My bad... Of course, you're absolutely right. In my haste, I thought that it was a 2S.... The FS should have been a dead give-away, eh???
In that case, there are 3 things that I would look for, in this order:
1. Open the valve cover housings. Are the pushrods actually connected to the rockers? I've seen where an engine took a rough landing and the pushrods came out of their sockets.
2. How is the valve lash? It probably calls for a 0.4mm feeler gage. There has to be SOME play in the rockers, but not a lot.
3. Has the cam been properly 'timed' to the crankshaft? You'll need an instruction book to find out what to look for in timing marks.
Again, this all presumes that all the simple boiler-plate things have been checked, a good 4-stroke glow plug (OS type F or equivalent), fresh fuel, tight plumbing, etc.
Just my $.02
Bob