have you looked at makerverse? and then shapeways or pokono? just a thought. there are people who do custom model work. since much time is involved, this is indeed spendy. but if you can get or make 3d files of the parts and shapes you need, these can be 3d printed. also in HO. there's litterally tons of scale detailing parts. more then any other scale or any other hobby. get yourself a walther's catalogue if you don't already have one. 1/87th (HO) is a bit small for creating something truly unique and origeonal. i'm not saying its not done or can't be. only that there are other and larger scales that in some ways it might be easier to. the proportion 1/87th is in many ways odd and awkward. the principal compensation for that, is that so humongous much IS available in it. like i said, get yourself a walther's catalogue, if you don't already have one, or go to their website. you may need only a few custom parts to 'kitbash' (use most of an existing mechanism, ready to run or kit, and modify it with bits and parts form others) either exactly what you want, or something very close to it.

ah you might also look at european model suppliers, the sentinal 060+060t, might very well exist as a british 00 bachman. or other mfr. then pretty much all you'd need is some paint and the face. and i'd almost be willing to bet, there's a source in HO/OO for the faces.