Speaking from physical size and fit or anything else I don't think I would call a YS91 or YS110 a 40 size engine. A Saito 72, 82 or a YS 53 or 63 are "40 size" drop ins. If you are looking for the kind of power produced by a "power house" 40 size 2 stroke, i.e. Jett 40/50, YS45 or even Webra 50, Rossi 45/53 etc, etc your not going to get it with a 4 stroke that is a drop in fit for those engines.

Denis