you have air trapped in the head. try bleeding out air at head and at waterpump ....or .you have a bad head gasket .. Hisuns are not for riding, they are for working on
I’ve done a lot of research on this swap and a good buddy of mine is an awesome ATV/UTV mechanic which gave me a lot of good info. hisun 750 (735cc) 38.22 HP Yamaha 700 (686cc) 50HP
That in its self is worth it!