I have a 1963 Evinrude fastwin 18hp short shaft outboard, model 18302, that I cannot get to stop mixing water in the lower gearcase. I've tried a new skeg (fin) new seals, ended up replacing the prop shaft bushings and seals as the original fin was cracked. I believe now that the fin and the main gear case housing are machine fit to each other and mixing means not matching, and not sealing. So I've decided to fix it once and for all, in finding a complete lower unit from another outboard. What years and hp models of Evinrude and Johnson have lower units that will swap out with my original? I do intend to replace all of the seals and gaskets on the replacement lower unit as well as a new impeller. As far as I know the one on there is original from 1963