I spoke to the mechanic involved in the e15 head a12 speedway outfit that rallycar1200 got me onto and he can do the a15 with the e15 head with modded almost de-counterway crank and modded journals to accept hayabusa rods, his specially balanced alloy flywheel to suit the lightweight (not knife edged) crank and rods and some off the shelf 80s motorbike forged pistons to suit the 2v ohc chamber head. He predicts 200+ engine hp from his a12 engine but a bigger 380cc and longer stroke crank could maybe be similar with more torque. Also he uses flatslides.
This of course is a high revving engine and maybe not as streetable as desired.
He can also do gearbox adapters for a series to toyota and nissan L series gearboxes.