The fan on any furnace is balanced. If in the process of replacing bearings, the fan got bent or you lost a weight (like on a tire), it would become unbalanced and shake. A furnace that old is probably in the 60-70% efficiency range. New ones are over 90%, so maybe a third more efficient. And, some of them are two stage and could have a multispeed blower that makes the overall comfort higher because the furnace can match the load.
If the blower motor is still good, you might be able to find an old tech that could balance the fan, but that is almost a lost art done by machines in the factory now, rather than by eye, hand, and skill manually. There are probably numerous motors that could be substituted. It would be hard to tell without the manual or having the old one in hand, though.
You might get lucky and find a compatible replacement but I'd think about maybe it's time to bite the bullet.