Thanks for the responses.

-I planned to have one carburetor (and crankcase) for both cylinders.

-I have access to a lathe and a milling machine but I don't have that
much experience with them so I'll have to learn as I go or get someone
else to do the machining

-I do plan to build the engine in a modular way so I suppose I could
add more cylinders.

-I do have some experience with lost wax casting although not with
much success. The engine casting that I had envisioned may be too
complicated for the simple gravity casting techniques I was planning.
I also worry about hot tears of the cylinder as the aluminium cools
and casting decent heat fins.

-For the cylinder I was planning on just an aluminium wall since I
have heard "cheap" lawn mower engine can get 500h on them. I though
that the bearings in the engine will probably need to be replaced in a
few hundred hours anyways, so the bore could be resized then.

Brock