Ian,
I think Tim NZ has nailed it.
Using a set of LATE classic Bullet cases is probably your best bet (if you can find a set), the main issue is the change in one of the timing cover oilway positions in the change from classic to lean burn timing cover. The later classic cases have the alloy "blob" in place which can be drilled to match the hole in the timing cover, while the other hole can be plugged.
My AVL/Redditch hybrid actually uses a modified set of old (90's?) 350 crankcases with an Electra-X crank, top end and timing cover, Henry Price who built it had to build up some alloy weld for the modified oil way but also managed to fit a wider timing side main bearing. It has also had Bullet Whisperer's ASBO tuning since, not essential but fun... I also fitted a set of Samrat wide stem cam followers from our hosts.
Ignition was by standard contact breaker drive for a 500 Bullet fitted with a Pazon ignition, though it has since had a BT-H magneto. Of course with a full set of timing gears you do have to compensate for drive gear backlash when setting the ignition timing, not a problem for fixed ignition.
Photos courtesy of Bullet Whisperer.
A.