Yes, the slant head is mostly for clearance. People say that it creates more compression, but I'm not sure how true that really is. Not saying it's not true, but if I were to look into increasing power, I'd look more towards expansion chambers, thinner head gaskets, and head milling/sanding. Those are things proven to work.
On my frame, the engine fit fine but from where I put the CDI the plug wire couldn't reach the spark plug and probably would at least have a difficult time doing so with a straight head. All I had to do was rotate the head and that problem was solved.
Rotating the slant head makes no noticeable difference in performance. Ideally, you'd want the plug pointing towards the intake so that the explosion pushes the exhaust towards the exhaust port, but on these small engines this doesn't make any difference. I had the head both ways and it ran exactly the same.