A mate of mine has had one of these on a big CNC router. I think his is
3 or 4 hp. The only problem he had was his own fault. He did not use anti freeze in the coolant and the body split. Made a tricky job for me clamping it back together across the crack and re-sealing the water jacket but it is still running!
I had got the idea that you'd already fitted a high speed spindle to the BCA. Hadn't it happened yet or wasn't it powerful enough?
I've still got a Mk2 equivalent that I inherited (looks like an Exel model 0, but no name plates, obviously WW2 clone manufacture). That needs a bit of work, starting with replacing the 7/16" BSW feedscrews and nuts!
I've toyed with the idea of converting it to 5 axis CNC but may well restore it to full manual function first.
I fitted a spindle made from an ER-20 extension driven by a (possibly overpowered) 650W DC motor which fits on the side of the moving part of the head, not at the back with drive cords. Faster than the original slow motor, but not particularly high-speed.
I also have an option to fit a 64,000 rpm air motor, but changing over is a pain and accuracy is not good.
I now need to do some very small-scale work, cnc-cutting turbine blades with ~1mm cutters in inconel, and I thought a high-speed (and hopefully lower vibration and TIR) spindle might help.
It won't actually be run very fast, carbide in inconel needs fairly slow SFM, but hopefully it should be better for that job than the present spindle.
I have also (3/4-) built a new up-and-down slidey bit, with new cast-iron dovetails which which run on the old dovetails on the fixed part of the vertical slide, so I will be able to just swap heads over as and when appropriate (just by undoing/redoing two screws and one plug :)
I replaced mine first with 10 by 2 mm leadscrews, I couldn't find any
12mm by 2mm leadscrews, I'd probably have used them if I could have.
Later I replaced X and Y with 10 by 2mm ballscrews, and a 14 by 2 mm ballscrew for Z. You need TDK-style triple-circuit ballnuts, the bodies of the other versions are too wide.
I'd think about using 12 by 4mm mm ballscrews for X and Y cnc if I was doing it again, but even with TDK style the nuts might be too wide.
I think I still have the almost-unused 10 by 2 mm bronze leadscrew nut pairs for the X and Y screws somewhere, if they might be of any use to you. IIRC I did a reasonable job machining them. You could get the matching leadscrews from Marchant-Dice, the ones I used were repurposed.
That's about what I did, except to a metric manual version first. The large turntable is surprisingly useful then, though it's not really useful for CNC (the extra room when it is removed comes in handy).
It's now (nearly) a 4 axis CNC.
Actually I'd quite like to have another manual BCA available, if you know of any going at non-silly prices (there was one went at iirc £1,800 on ebay not that long ago). Small envelope, but otherwise fairly capable once the feedscrews, spindle etc are replaced.
(the spindles are good, but the collets... easier to change to an ER or compatible spindle)