Where's the origin phone?
If you can, get a Pre1 and confirm your Sprint modem is still functioning well. Shouldn't be hard to find one cheap if you don't have its original host, or pay a little more for a good condition model so you can use some parts, like the screen and keyboard.
Once you know your modem is good, either a VZN or GSM Pre2 will work for the Franken surgery. But you will want the CDMA sub frame for the antenna.
You could put your current Pre2 motherboard into the donor Pre. It's more work but doable. You'd downgrade only the camera because you'd need that cdma subframe.
As far as your comm issue, I'd sooner expect the antenna is suffering contact corrosion, before an internal board failure. That thing is sealed up pretty good.
But - If your comm board IS bad, you could try to get a Sprint tech into adding the replacement board (from another Pre) to your account and activate THAT Pre. THEN do the surgery to combine your good MB with the newly activated com board, and your best condition keyboard, screen, and sub frame.
I've tricked them before, getting them to update my account with the numbers from a replacement phone, so i didn't have to deal with them trying to charge me for a phone change. It requires a very helpful tech, not the first response CS rep.
I wish i had spare parts to send you but i used up my supply!
P.s. before all this, try a complete reset and doctoring. If it miracle-cures everything, it would be a lot easier to restore your software than disassembling hardware.