Dont drive it until the motor blows, that is a nice car. Remove the head and take it to a mahine shop and have them cut out the old valve seats, and press in the new oversized strengthened ones. Maybe peen them just incase. The local machine shop where I live even had a set of four in stock just because he sees them a lot.
It will be a lot cheaper to just have that done right now, then dealing with damage that occurs from the seat dropping. The SPI is a great econo motor other than the valve seat thing.