Adrian...As long as they go into the primary case and are a good match for the diameter of the original screws, you will have no problem. The secret to an oiltight case is a good quality gasket and smooth mating surfaces. My favourite jointing is Hylomar, purists will use Red Hermatite. Silicone bathroom sealer if that's all you have.
One of the screws acts as a level plug, one as a drain, so these will need a fibre washer under their heads to act as a seal. The oil level is quite critical. I run the oiled chain "dry" and add oil until a nice stream of drops passes the filler hole as the motor ticks over.
This means that now the chain is running in oil, but the oil is not deep enough to get into the clutch.
The screws you have will be fine, they may actually be made for the job, rather than some generic fastener. They only need a firm tighten, not the extremely murder tightness that results in knackered screws when you try to get them off.
Swarfy.
