Is this a working model Richard ?
Does it have to actually power the car?
How much detail are you looking for?
If you just want something that looks good and does not work, I would get a lot of photos of various JAP engines , pick the one you like the best (or is the simplest to model) and start using woodworking skills, I would guess you need to start with the crankcase , then the barrels , the cylinder heads, then all the detailed stuff like push rods rockers , carbs , exhaust etc. I imagine some of this could be done on a 3D printer if you have access.
I think that Cameron Engineering produce new JAP replica engines at full size, perhaps you could get some inspiration from their website.
