Looks like you are stuck with me again judging from the lack of respondents to you recent question, but then that seems to be the way of the forum right now.
How much would it cost, well how much are you prepared to spend, throw enough cash at the project and just about anything could be achieved, but I guess you would like a bit more input than that?
In the absence of mates to do a bit of turning, milling, grinding and welding on the cheap or on the, you help me and I’ll help you basis you then have no alternative but to go commercial, for commercial read pricey!
Before any of that you will need to establish the concept and the design of the engine; air cooled iron or aluminium, water-cooled, piston port, or reed-valve inlet. Will it be a square bore and stroke, or short stroke with a 56mm bore, you mention short stroke so that precludes a 175 engine with the 58mm stroke. The most complicated design spec and therefore the most protracted and costly would be a water-cooled, reed-valve set up, however, this is potentially the most powerful and user friendly option.
For instance, a cylinder head casting will obviously be a simple engineering task to produce if it is an air- cooled one but the water-cooled alternative will be a two part design, with double the cost. Will you make your own crank assembly, install larger more robust main bearings, convert the gearbox to ball races and needle rollers, and make a new clutch basket for extra plates maybe even get the assembly dynamically balance? And so it all goes on, you may have a handle on all of the details but try explaining it all to a guy that perhaps has no idea of what a racing Bantam is. It is so essential to offer him a decent set of engineering drawing; the universal language.
If you are forced down the commercial route then you will have to have sensible drawings produced to a reasonable standard. When I started my final engine I spent a long time getting it all on paper, an exercise in thinking out loud, so worked on the basis that it is far better to rub out a few pencil lines on paper than have to remake an incorrectly dimensioned or ill- conceived component! I also had the great good fortune to have the tacit agreement with my boss to have free use the entirety of the available engineering facilities where I worked, sadly the CNC machines were in continuous use, I could have made a far better, free flowing job of the exhaust duct with CNC production, but I managed.
Personally I would not undertake such a project under the conditions you describe It is likely to be prohibitively expensive. Far better to buy an existing racer, chuck away the junk and work with what is left, or invest in a race ready RS Honda and get to the track straight away, the Honda could eventually be far cheaper!
If it would be of interest to you Les I could email a few of my own design drawings and pictures to illustrate the task involved, If so, PM your email address and I get on the case for you.
Cheers for now, regards, Trevor