The engine maybe a brand called Zenoah ,I don't have a clue what size it is ,the 4 bolt head ,that usually means
that it is high performance ,or bigger than an average 23cc engine ,maybe 29cc an on up to a 32 or 36cc!
It is hard to tell just by looking at a pic ,if the size or a # is not printed on it ,then you may have to take the
head off an measure the piston size with some calipers ,the engine also could be modified with bore up ,
an special crank ,the only one that knows ,is the original owner ,or an engine builder!
Its a beautiful well done boat!....