Got the engine case back from the welding shop with the top and bottom bungs TIG welded in place. I cleaned up the welds with my Dremel tool. Here's how it looks with the plug and drain installed finger-tight.
Also put it in the frame for a trial fit with my fingers crossed to see if there was room for the check valve. Looks like everything will fit.
In the next several days I'll get the motor properly assembled and bench test the intake valve setup with compressed air and see if I can get it to run.