Re: Making a custom bike engine
Theres much better ways of starting a project as ambitious as this. You'd be way further ahead starting with a full rotating assembly as others have mentioned and building a block to match.
There's a ton of great inline 6 engines to start with. Chev and ford inlines are bulletproof building a cross flow head would be a great project to learn with. If your looking for small bore as I assume you are honda inline 6 from a Nike would be a good donor. You could use most of the engine but make new cases to mate it to a bmw trans for a shaft drive. With rb26 and 2jz engine readily available there's a ton of great oem and aftermarket parts available for adapting to other engines as well.
If your really that ambitious starting with a gx200/ gx390 or clone would be the cheapest and probably best option for a decent engine.