My little clinker ply boat is not dissimilar to the OPs (just more elegant ) ) And while building her I seriously considered a little Honda stationary engine with a retractable prop unit. When retracted the hole would be faired by a hinged door. Lots of measuring and lip pursing later ( and time constraintes) I just put an outboard well in. Actually I never take the o/b to sea, just use it when on the river. The first idea makes RR's look positively rational.
Good luck with the ST Steve, I am with you there, two in pieces and one complete right down to the mounting plate , all waiting for a rebuild. Got two grandfather clocks and one French clock to do first though.