Fireman,
How deep is the Spacer Walker cowl? About six inches? I think that the shaft can stick out some since it doesn't use a large spinner. The Quadra is about 6" like the Homelite depending in the mount used, but much bulkier. It would give the best performance of the cheap choices. The Ryobi is 7" long with the carb enclosed in a mount or standoffs. If you make a hole in the firewall for the carb, it can be about 5" or so. Since they are conversions, the choice of parts and type of block determine the exact lenghts. If you could get a good running Quadra 35 or 41 for under $100, it would be great. The plane has a short nose, so the 4.5 lbs of the Quadra would help balance. The other two conversions will weight about four pounds on magnito ignition anyway and swing small props. The light weight high dollar choices, could be easier starting, but will just cost more in this application.

Dave