How to plumb a standard water heater tank with my rooftop panels?

Try Sunearth solar products. They have 65, 80 and 120 gal. tanks. I believe they are rebranded Rheem tanks and plumbed for solar. That's what I replaced an old tank with on my solar thermal system several years ago.
Also, Home Depot sells Richmond 80 or 120 gal. tanks. 80 gal. price is $1252.00 that are probably the same thing as Rheem and are plumbed for solar thermal.

The Rheem tanks are still made but you need to search for someone that stocks them. That said, unless the system was designed for potable water which means no lead solder or fittings containing lead you will need a heat exchanger to keep the outdoor fluid separate from the potable water.

Thanks for both for the good info. The system was installed in 2004. How can I tell if the outdoor fluid is separate from the potable water? The tank is a Lochinvar FTA120K, so I will have to call them, right?

Call them but I'd suggest calling others as well. Phone calls are cheap these days.

You most likely have what's known as an "active", "direct" system meaning it has a pump (the active part) to circulate the potable tank water through collector loop without using a secondary fluid and a heat exchanger (the direct part). That's what I have.
That means that the water in the storage tank circulates through the collector(s) on the roof and back to the storage tank without an intervening heat exchanger.
So, the same hot water you drink and shower with also goes through the collectors before it gets to you.

If you did have a heat exchanger it would be obvious and external to the tank. If so, you'd most likely have another pump to circulate the fluid through the collector loop as the tank you have does not have an internal heat exchanger. If you do have an external heat exchanger you will also have a secondary working fluid, most likely 50/50 (propylene) glycol/water mixture. but, as you write with no freezing problems, you have no need of that and besides, if you did, after 20+years in service you most likely would have changed out the glycol mixture several times. If you haven't, you've either got big problems from acidic glycol or an active, direct system.

What you call the "outdoor" fluid is in all likelihood potable water from the tank.
Do a bit of homework for details on how different types of solar thermal collectors work.

As for potable direct systems and lead-free components, I had my tank replaced several years ago and the plumber used lead free solder and fittings, all available and mandated in CA. That costs more but I'd guess the same or similar rules and availability apply in FL.
Working with lead free solder is however a pain in the ass to some plumbers as it has a higher melting temp., needs a lot more flux (messier) and the finished product may look unusual or different than what the old lead-based solder looked like and so cause some suspicion about quality of the joints.

I'd shop around for the tank. Home Depot sells the 120 gal. model for ~ $1,900 on line. Others charge up to $4,500 or so.
My strong suspicion is that regardless of the name on the tank, they're all made in the same place and supplied to major marketers.
These tanks are a commodity.

Lochinvar (who I suspect to be the largest original manufacturer), Rheem and A.O. Smith sell them although the latter two probably get them rebranded from Lochinvar.
Outfits like Home Depot (@ ~ $1,900 ) and Solar Direct (@ ~ $2,900+) also sell them. Prices are most likely plus tax and freight.
Not a plug, but see the Solar Direct website for a cartoon of how your system is most likely plumbed.

With so many of us 65+ YOs still roaming the earth after drinking water from lead soldered copper pipes & with lead pipes used for the water service from the street is it really a concern especially being the hot water side if the tank is lead free?

Well, concern is an individual decision and while I'm probably in more than a little agreement with where that question might be coming from, I'd think the answer is more involved than can be discussed on a forum that has "SolarPanelTalk" as its masthead.
Anyone here from Flint, MI ?

Thanks to J.P.M. for lots of good info. You are correct, Home Depot has solar plumbed tanks for around $1900. Also correct in that I have an "active, direct" system. I am currently looking for a less expensive option, or a way to plumb a direct system into a standard, inexpensive tank.

You're most welcome. As someone who's so cheap they take their glasses off when not looking at anything, I appreciate your opinion.

However, as for using a std. tank and making it work in a practical and efficient way, that's tough to do in a workable, practical way with only one tank.
Problem is a std. tank doesn't have enough holes in it to do what's needed and do it the correct way without a lot of piping shenanigans.
It can be done but I doubt you'll find a plumber who will tackle the job even if (s)he did (?) know what's required and - no offense - reads to me like you don't either.
And, even if you do, chances are the extra labor and materials you spend to make a cheaper tank job work will eat up most, all or more of the difference in price between a solar specific tank and one or 2 smaller (but equal total capacity) and/or all the extra labor and material.

Looks like what will work for you is $1,165.00 on line at the orange big box. If that price is correct, it's about as cheap a 120 gal. tank as you're likely to find at this time. Check it out.