Quickest and easiest answer would be the TLS 7" brake from the late 1960s bikes, which is excellent. Second best would be to get the existing brake relined and properly set up. It's a myth that these brakes are no good, and likely comes from the era when they were cheap hacks used by hard-up apprentices unable or unwilling to spend money on them.
Even new shoes, heavy duty cable and a good setting up would doubtless improve things immensely for you.
Fitting a disc front end ain't easy, and now you have to inform the insurance company of any major modifications, it can all get complicated very quickly....
to be avoided if possible.