Engine Decisions...

I've considered using a Lifan or Piranha engine in the red SL90, but I needed to know if a Lifan or other clone would fit into the SL90 frame without significant modification to the frame.  I don't want to destroy the originality of the frame, so I needed to know how a Lifan would work as they are advertised as fitting into the SL90.  I've seen many Lifan transplants into other vintage Hondas, but not the SL90. Fortunately, after searching and contacting vintage Honda forum members, I was able to obtain photos of a Lifan transplant into an SL90 to help with the decision process. 

The Lifan fits, but you must drill new holes to mount the engine base to the skid plate.  Also, one of the rear engine mounts will need to be cut (see below) 

Finally, the upper engine hanger brackets don't line up (below)

Given the extent of the necessary modifications, I decided to stay with OEM stock Honda SL90 engine cases.  I have two SL90 engines - the original, with the correct VIN for the frame and a donor engine for parts (see below).  It will require a complete rebuild and I can bore out the cylinder and make some other improvements without having to modify the Sl90 frame.

The pitiful donor engine & sad original engine cases