I have a 91 NA FC body with standard tranny and bare block. I bought a 90 TII that had crashed. I plan on putting the engine and tranny from the TII, into to 91 body as well as the interior.
Anyways, I haven't gotten deep into the engine of the crashed car yet, the frame from the passenger side fender had all been pushed in, as well as the front bumper, only just hitting the engine and breaking a couple radiators up front, but the engine still runs, although it seems the MAF meter had been destroyed. How important is that part? what is its function exactly?

Also, since there are gobs of wire in these cars, I wanted to know if I have to use the wire harness and all from the TII, or can I use the Harness in the NA?

Any other general info on doing a swap like this would be helpful, or a link. I tried to search, but mostly it's S5 to S4 that I see.

 

The MAF calculates the amount of air that enters the engine, so that the ECU can calculate the amount of petrol it puts on the engine allowing for a correct combustion.
If the MAF is not working or not present at all, the engine won't work correctly, it will use more fuel than necessary and the performance won't be as good.

About the Wiring harness, here you have 2 topics that talk about that:
https://www.rx7club.com/2nd-generation-specific-1986-1992-17/na-wiring-harness-ecu-turbo-ii-motor-413482/
https://www.rx7club.com/forum/showthread.php?t=298878

And about the Swap, there are some topics already here on the forum that talk about that just search and you will find some for sure.

 

See the 2nd gen FAQ for turbo swap information. It is the single most-covered topic for 2nd gens. NA harnesses can easily be adapted for a TII engine, and yes, of course you will need to replace broken parts.