Hi lordy, there may be other options depending on how old your motor is, if it is old enough to be power rated at the flywheel then you would probably get the same performance out of a more modern 70 that is rated at the prop. Regardless 5 hp is not going to make a dot of difference to your transom if the transom is in good condition and you have no complaints about the 85 on it, what will be important is weight more-so than than the HP you are thinking of fitting. Nq