Just to help with a diagnosis, when the brakes didn't stop, did the pedal go to the floor ?(usually a fluid leak / MC failure) or did the pedal suddenly become unresponsive & very hard to press down? (failure of the power-assist)

AS for changing to the later, dual system, to install the correct integral dual MC/vacuum-booster would require major firewall surgery, moving the battery and a different pedal assy & clutch master cylinder.
Alternately, one might be able to substitute a dual master cylinder without the booster, and plumb in a PAIR of remote vacuum servos - one for each circuit. Seen this setup on a late '60s British Rover 3500.
The longer, dual master cylinder might still require moving the battery.

Happy Motoring, Mark

__________________
DrDKW