for this stuff, a 7" lathe will do everything needed on a conversion, doesn't have to be completely accurate since it's mostly one off stuff... I got my lathe for around $250 on craigslist lightly used (they only retail for around $400, but still that's a good deal), it'll make pretty much anything I need on conversions, I've made spark plug adapters, prop adapters, custom standoff's, I've machined the crank some to convert the dang left hand threads to right hand and turned off the rear crankshaft section. most of the rest of the stuff you want for conversions can be done with a dremel and hack saw, unless you want it to look perfect... remember, aluminum doesn't weigh too much, so you have to remove a lot of it to make it worth while, steel on the other hand is heavy...

I've seen the mill/lathe combo things on craigslist for around $600, and a couple like mine for similar prices to what I got