jono not sure about engines but freeboard at the transom depends on the surveyor doing the coding i had a customer who had to put in a false transom ,all he did was fix the new transom /splash board to the a-frame with u bolts and sealed the bottom and sides to the tube and deck with silicon so that it was easily removeable
hi jono i've seen 21's with twin 70 and twin 90 tohatsu's. i think there was one for sale some time ago with twin 115 4st yams, stand to be corrected on that as it's only a vague memory. if it's of any use to you i can measure the transom and engine centre's on a 21. a divisonal engineer told me some time back that early 21's had weak transom's that gave trouble when upgraded from 50 to 70 rudes hence large transom to floor brace.