Another possibility Matt is to evenly space the A series chamber
rather than offset chambers like factory versions have.
This will allow full 83mm sleeves on any A series as mentioned
by Lemonhead machinist who sleeved and bored it out to 82mm.
83,5mm was the max safe possibility but no room for rebuilds.

Alternatively flat plane A12 cranked ohv v8 with separate crankcase
and cylinder case to save on billet sizes. A12 rods are way wider
than todays v6 engines by almost 2 fold so custom bike ones could
share the same main bearings 3L ohc v8 would be sweet.