I agree that the OHV Honda is the best available engine. The 20" SB45 is relatively uncommon and I don't recall the original engine - the 14" began with a Briggs and later unfortunately switched to a Kirby-Lauson, while the 17" began with a Kirby Lauson and later switched to a Briggs. Since you are going to use the machine regularly, IMO the best solution would be to fit the GX160 and keep the original engine in safe storage. However the original is long gone and presumably untraceable, so you can only do part of that.
