Massimo MSU800 engine not running properly

[Bigal]

My son purchased this 800 because it was the right price but it has issues. I was just rebuilt with 10 to 20 hours on the engine. When it was rebuilt they put a new crank and bearings, new water pump, new oil pump gear, new pistons and rings.  It cranks slow like no compression release of a spun bearing on the crank. When it starts it will idle for a while then sometimes just stop. Other times it will idles and not stop but when you go to give it throttle it stalls and if it was idling for more than 2 minutes it's hard to get started again because it cranks so slow. When it's idling and you give it throttle slow enough to get the engine speed up it's loud. I did check compression it's 125, I did check the #2 cylinder valve lash but not #1 as of now but I will one day this week. Has anyone had similar issues with this engine?