Nick,
There really is no manual that I know of for conversions. There are however good or easier engines to convert than others. Any engine that has to be cut out of the saw, trimmer, or whatever it's from will be more difficult. It is nice to do an engine that requires no cutting and hacking of excess case material.

There are a good choice of engines that will mount like a glow engine, making mounting much easier and avoiding extra fabrication. Some of these engines are:

Poulan 46cc
Echo 44cc (same as a BME44)
Homelite 45cc
Homelite 50cc

I know the Poulan 46 is very popular and easy to convert, not to mention powerful and cheap. It all depends on what you have at hand. Did you have an engine in mind? That may be a better way to get your answers. Asking questions about a certain engine will usually result in some good replies.

Tim