Agreed----$100 as is.----These are nice running motors if in good condition.-----Shops do not work on them , so they are no good to modern folks.----Folks who know how to work on them have a few already.
Very common issue.----My guess , you took lower unit off in neutral.----If yes I would say it jumped into forward when you were not looking.----Start over and make sure lower unit and control are both in forward or both in neutral when assembing.