will a 70hp lower unit replace a 90h LU??

i need to replace the lower unit on my 1988 90etlg. there is an ad for one from a 1987 yamaha 70hp. thats all the info in the ad. will these years and horsepowers match up? if so what else do i need to find out to make sure they will match?

thanks,

[boats.net]

They should match up.