Matt, Will,
great info! I just picked up some Korean made glass (FRP) mounts which have an interesting lightened truss like design. They are two part mounts so RE headers are not an issue. They are quite versatile in terms of fore aft engine position as they have "slotted" holes so locating the engine for the spinner distance is a simple matter of tightening the bolts in the correct position. Once mounted for a 52 mm bolt width, most all 60 size engines would fit so it would be easy to swap at will. The disadvantage if say is that they may result in some power loss compared to a hard aluminum mount. I'll post some pictures later.
Matt, the OS 4 banger 90's have the same mount dimensions as the 2c OS 60's. But you are probably right about the YS 2c and 4c. The latter might also be 25/52 mm spacing.
David
Last edited by doxilia; 11-15-2014 at 01:03 PM.