You could possibly cut a new o-ring from a piece of neoprene tubing, I think you do need a seal there.
The newer one has the three topmost fins of the crankcase with a larger diameter, and the alignment pin for the cylinder is also solid. I've test run mine on diesel, with a glow plug and all shims removed, it felt quite alright but the o-ring couldn't take the fuel.

Here is an image of the latest model: