Die Cast 049's

There was one cast case engine model with an integral tank, namely the R/C Bee:

http://coxengines.ca/cox-.049-rc-bee-engine-2.html

Maybe you could source a used tank or a complete worn out engine cheap somewhere?

I have no idea how these tanks perform compared to sure start backplate. I have one complete (I think) tank but no cast crankcases. I compared the reed retainer area of the RC Bee tank and a regular one, the difference in the diameter of the two is about 1.5 mm, and the round part has two flats, presumably to make room for the piston to reach BDC.

Maybe, just maybe, you could make your circlip version of the bee tank fit by getting one of these kits:

http://coxengines.ca/cox-.049-engine-tune-up-kit-die-cast.html

The idea is that you would take the red part and use it as a spacer between the bee tank and the cast crankcase, and in this kit you will get the correct cast case gasket too. Just use the original circlip to hold the reed valve. Please note that this is just speculation, I do not know if it would work.

I'll see how much the shipping would bee for the RC Bee tank if you are interested.