No, you’re not stuck. You can use a Cognito front hub conversion though they are mostly for USD modern forks, that rules out the Joker triples. You can also use an old HD front hub, laced to a new rim that accepts discs. There’s some other spacers and caliper brackets to fabricate, but…
I have a 550 that I bought Joker triples for, but had to shorten the stem as it’s 0.780 too long for the 550. I bought new Forks by Frank tubes and used my stock hubs, upgraded rims and spokes. Single stock disc. It’s ok, but nowhere as nice as modern forks and brakes. All depends on how you intend to ride and use the bike whether the investment is worth it. For a stock street bike, used occasionally, I’d stick with stock setup, better tires and pads, and just get nicer rims (Sun by Buchanan’s). There’s later model 750s that had dual discs and a disc out back too, so you have some stock options and can still run the Joker triples.