I know this is a Zombie thread from 2015..
But, as a bike mechanic, and for general knowledge base, the only way you'd be able to use a 3 piece, cottered crank of his type with a CG engine is to get the crank arms off the spindle, put the cranks in a big f#ing vise, get out a MAPP torch and heat the **** of the crank arms just an inch or so above the cotter holes and beat it outwards to about 10 or so degrees each side with a decent sized sledge hammer.
You'll then have to re-bend the section near the pedals to keep the pedals from wobbling. Rinse and repeat. It's what I had to do on my Steyr built J.C. Higgins 3 speed conversion, motorized bike. That particular bike in the OP's first post is a pretty rare beast. It's not your typical English threaded BB shell you can just slap an alloy crankset into.
It's a rare, European spec, 3 piece BB with one piece type cup shells pressed to the BB shell. These are smaller than American sized BMX and Schwinn type one piece cranksets and will not interchange. You're not going to find these BB's new anymore except maybe as NOS bits, or in used bike shops, or ordered from former Eastern European country's bike shops.