I'd be very interested in seeing what options there are for a front brake upgrade. Coming up with a bracket to fit a different caliper wouldn't be too difficult. Like you said the problem would be trying to find a rotor to fit & that would have to be necessary because 250mm isn't cutting it. Need at least 300mm to get any kind of leverage for a single disc. Then there's the caliper. I don't think the caliper you show would work...think it would have to be one with piston/pistons on one side only, doesn't look like there would be enough clearance between the back side of the disc & the spokes. The mid-late 90's VFR750 calipers you suggest might though.I noticed some vintage racers use an ap racing caliper, on a homebuilt bracket. But the one thing I am trying to work out is what disc to use? they look like they use the cb350G/360 inner (black) part of the rotor, but bolt an outer from a newer bike on. not much information out there on the web about disc swaps though :\
I was thinking a VF750 caliper might fit an (twin piston) be sufficient stopping power for the 350. looks like its a lockheed item from a ducati or triumph model : https://www.ebay.com/itm/Racing-All...m25f808e3d6:g:HjQAAOSwfIxZZ~Ap&frcectupt=true