Hello, I recently acquired an '85 Accord with 80K miles on it, it was running fine, I did a tune up on it, drove it for a bit and now the transmission is shuttering in 4th, mechanic says I need a new transmission, any suggestions? During the tune up I did drain and refill the transmission fluid and all fluid levels are at the proper amount. There is an automatic transmission on ebay for $300, how hard is it to swap out?

Thanks