I don't know about why you can't legally put another motor in there other than a 98. Heck, I put the engine block from a 2001 Taurus in mine when I had the same situation present itself. How would anyone know?
Depending on what's wrong (my problem was with the block), you could do what I did (see sig). It's not any cheaper, and it certainly isn't any easier. And doing that requires more work than just dropping the block in there. Those reman "G VIN" motors are definitely not cheap - I shopped around and the absolute best price I found was $2600 from a local shop that I'm not sure I really trusted (might be hot).
The situation totally sucks, and there is no good way out. Sorry 'bout that and welcome to the club nobody ever wanted to join.
