I owned a Cub in the early sixties & it was a fun, good looking bike that I'm quite familiar with. Upgraded to a Bonneville in 1963. Motors got the vintage look for sure. The original question isn't about a replacement but creating an affordable alternative to an expensive replacement engine. To lend a feel of age to the build, but not to fool anyone. The beautiful China Girl mod posted earlier is one that when I first saw it I did a double take and lol knew what had been done and I admire that creativity. It feels at home in the drop frame...beautiful bike as well.
Peashooters were mid 1920's factory team creations that competed through the 1930's in individual racers hands and were primarily used on the flat tracks (post board track era) & were a modification of the loop frame by Harley factory team bikes that cut the loop out and used the single cylinder engine as the lower support member of the frame & yes the Peashooter had a transmission (not unitized) as time progressed and it raced on the flat tracks.
Silverbear uses a phrase "building in the spirit of" which is so appropriate for the bikes I build... and I make no apologies for. I'm not restoring or copying a specific year but trying to capture a feel of an era, which I don't display but actually ride...often. So safety & reliability are also of concern when selecting an engine etc. Remember, and I do, trying to start that little British engine (when it was relatively new) on cold Oklahoma Winters & those hot restarts in the Summer...wasn't so fun then and at seventy years of age I don't feature it to be enjoyable now. Time hasn't diminished those memories! The CG 125 OHV basic design is truly dependable & more than 4 decades old which makes it more of a "true" classic than most of the engines available for moto bicycles & at 125 cc is a smaller displacement than most of the Predator style clones being used on 4 cycle builds today, which I also like but they don't look any more vintage than the CG Honda clones.
It all boils down to what appeals to each of us and that's also the way it should be. Build on and have fun!
Rick C.