I just want one as close to Vietnam era M14 with as many USGI parts as I can get on it.
My advice since you are asking, is the following:
1. Buy the nicest walnut or birch USGI M14 stock that fits your budget (with M14 selector cutout and buttplate).
2. Buy the nicest USGI handguard that you can find (brown or the slotted version if you like that look).
3. Buy a faux M14 selector kit from Forceman (to fill in the 'hole' on the M14 stock and make it look like an M14).
4. Buy a nice vintage green web sling as finishing touch.
With those 4 parts alone, you will transfer your rifle into something that looks just like an M14. I would not bother replacing other parts with USGI parts until I had changed the furniture, added the faux selector lock and a web sling. You might discover that those 4 parts are enough. The fake M14 selector lock is a chore to install, but that one item really makes it look like an M14. The stock is the most expensive part if you really want a very nice vintage USGI M14 stock, but the others parts are quite affordable. My 2cts.
Note: This swapping of “aesthetic only”parts will also keep your SAI M1A’s lifetime warranty in place. I’d keep the original plastic stock and hand-guard - just in case you ever need to ship it back for a repair under their warranty.
Fwiw, this is a 1981 SAI M1A that I bought around Christmas. The only USGI parts were the 1) stock, 2) handguard, 3) rear sights (M1 Garand lock-bar sights), 4) hammer, 5) trigger, and 6) vintage web sling. Just adding a faux M14 selector lock to this rifle - and it will then look almost exactly like a Vietnam era M14. Those are the 4 key aesthetic pieces. So that's something to keep in mind...