Welcome to the forum! I can see by your question that you must be a resident of our lovely Sunshine State, but perhaps you would like to introduce yourself in the Newbie Check-in forum and tell us a bit about you.
I can tell you when you buy a bike and get a title with it, all you have to do is take the title (filled in and signed by seller(s) and purchaser(s)) to the DMV and show your driver license and pay about $120 + sales tax on purchase price of the bike and you will get a tag, title in your name, and registration.
Now, taking a frame and essentially putting a different motorcycle in it...I'm not sure. You said you already have a title on the frame. In my case, they didn't ask a single question or need to see the bike to get it registered. You may very well be able to just take that title and get it registered and get a tag. If you try to sell the bike in the future...that could be a totally different matter. And if you get pulled over for something and the officer decides to be very thorough and compare VINS on the frame and engine...that could be an interesting conversation.
If you want to do it the right way, I would suggest calling the DMV and asking them.
I did it, but I got a destruction title (after I paid it, online , The broker company ask me to sign a power of attorney to get a new title) two weeks later I got (parts only title) DMV has nothing to do with it. I Asked. The bike runs, I just want to ride it.
Really shouldn't be a problem. Folks stick new/used motors in frames all the time. If you have a title to your frame and put a new/used motor in it there be no problem as long as the motor isn't reported stolen.
I put a used motor in my car and I still have the same VIN number. Legally if I sell the car, I have to show the reciept for the motor to the buyer, but the DMV doesn't really care if the motor is different.
But, Florida may have different laws, so don't ask us, call the DMV there and ask them.