The square "socket" on the engine, into which the square end of the flexi-shaft fits, is (I assume) on the output end of the clutch assembly. If you remove the clutch assembly you should find that the output end of the crankshaft has a threaded hole in its centre. A threaded stud can be screwed into this hole, onto which a suitable coupling can be screwed.
An IC engine should ideally be on rubber mounts for noise suppresion, thus a rubber-bloc type coupling should be used. As there will be some movement of the engine on the rubber mounts, a propshaft assembly with a ballrace on the engine end of the shaft is really necessary.
I have a 24cc engine from a "Talon" (Chinese) strimmer. I've been told that it's the same engine as a McCulloch. The threaded hole in the crankshaft is 5mm, to which I've attached a Prestwich Models coupling. The proptube/shaft assembly (home made) has a bearing housing on the inboard end with a ballrace fitted to suit the 1/4" shaft. Shaft and tube are lubricated with ordinary bearing grease.
I have fitted aluminium plates at each end of the engine with four 15mm x15mm rubber blocks at each end. Outer aluminium plates are fixed onto the blocks, then these outer plates bolt onto aluminium tabs glassed into the hull.
I don't have any pics of the engine installation to show you, I wish I had, and I don't have a digital camera. A picture would explain the installation clearly.
I don't have any experience of running such an engine in a boat with the clutch still attached. I think it would be more trouble than it's worth. Modified CMB.91s churning out 7.5bhp are run on direct drive with no problems, and strimmer engines are nowt near that output.