Here is the illustrated parts list for your engine:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6ntABVJ1DajI.pdf

I can confirm that the standard thickness Crankcase Gasket for your particular engine is P/N 270833.

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/flivAJX-nfBhU7y.pdf

Your engine was made on 13 February, 1990. That is long after Rover took over Scott Bonnar in 1980, but if I recall correctly the Scott Bonnar plant continued to operate for some time after the takeover, before Rover moved all production to their own plant. That is a matter for Deejay to clarify.

I think the next step is for you to check the tappet clearances, in case you interchanged the intake and exhaust tappets when you reinstalled them after they fell out when you removed the camshaft. To do this you will need the crankcase cover installed - do not tighten the screws since that would lock up the crankshaft. You must have the cover installed though, because it supports one end of the camshaft, and the camshaft has to be supported for the valve clearances to be correct. Look at this illustration from the parts list:
[Linked Image]
The red oval shows you where the valve cover/breather attaches to the engine, directly below the muffler and carburetor. It is attached with two screws, and has a breather valve built into it. Do not touch the breather valve when you remove the valve cover: it is fairly delicate. Note the black gasket which keeps the cover from leaking oil: do not damage it - if it becomes damaged it will have to be replaced.

Look behind the valve cover: you will see two coil springs, and just below them, the junction between two metal parts: the tops of the two tappets, and the bottoms of the two valve stems where they emerge from the coil springs. To see how the valves operate, pull the starter very slowly with the spark plug removed: you will see each valve in turn rise and fall. Turn the engine to Top Dead Center at the end of the compression stroke - both valves will be closed. At TDC or very slightly past it, you need to measure the valve clearance, which is the very small gap between the top of the tappet and the bottom of the valve stem. To do this you use a set of "feeler gauges", which are very thin strips of hardened steel. Each gauge is marked with its thickness. The intake valve clearance should be 0.005-0.007". The exhaust valve clearance should be 0.007-0.009". If both clearances are about right, reinstall the valve cover, ensuring that the engine still turns smoothly and freely afterward.

If the valve clearances are correct and the camshaft timing gear mark aligns exactly with the crankshaft timing gear mark, you are ready to reinstall the crankcase cover with its gasket.