Since the front end is already non stock you have a world of choice.
The forks look OK, , if you know a man with a lathe pretty much any other hub brake will fit, these range from super spendy double sided multi shoe jobs, down to the later 68-70 8 inch TLS which is fairly common and very effective, somewhat heavy though.
If you change the front end entirely then the cheapest possibly would be some sort of oriental set up with a spoked front wheel and disc, Honda Hornet or somesuch, look at Hillbillys bike, it does the job. pretty much anything can be adapted to fit, but will need custom steering head bits to make it all play nice, lathe work again. later triumph bits will fit relatively easy , and have convenient rubber mounted H bars which fairly take out the dirl.

71 Devimead, John Hill, John Holmes A65 750
56 Norbsa 68 Longstroke A65
Cagiva Raptor 650
MZ TS 250
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