An old flat head engine would fit better than the slanted cylinder OHV engines. You could use 3hp, 3 1/2hp, 4hp even 5 hp Briggs or Tecumseh engines. Depending how big/small your boy is a 5 hp might be a bit dangerous. With the seat overhanging the rear axle, the bike will wheelie a lot if a torquey-powerful engine is used. A 3hp Briggs will give slower acceleration to avoid the wheelies yet still go about 25-30 mph. Want some grunt? Put a 3 1/2 or 4 hp Tecumseh on it
Want wheelies and quick acceleration? Use a 5hp.
(just my opinions, others will vary)
Danford1
PS. If you have a welder and can fabricate, you can lengthen the frame, change the muffler and use the engine you have if it runs well.