The screw cap is there because you wouldn't be able to take apart (or assemble for that matter) the engine without it.

Procedure:
1: remove cilinder head
2: remove backplate
3: remove screw cap in the back of the cilinder
4: remove the cilinder sleeve
5: remove the wrist pin through the hole in the back of the cilinder, and subsequently the piston
6: turn the crank until the conrod has cleared the groove in the bottom of the crankcase, and then take it off the crankpin and remove it through the cilinder
7: remove the crankshaft and bearings

Assemble in opposite order... and leave it alone unless it stops running...