It includes guidance from loudhvx (RIP) and Scirocco for using MCs from other bikes.
I set up a search on ebay and kept after it until I found a good used MC and an NOS internal piston. This genuinely took 3-4 years of absolutely no results, then those two items popped up within a few weeks of each other. Can't predict how that's going to go.
In the interest of time, you could adapt a part now and continue to search for the correct one. Either way, finding a stainless brake line to fit shouldn't be a problem at all. You can also make one yourself or have a brake shop make it for you with the fittings you want.
1979 KZ400 Gifted to a couple of nephews
1967 Yamaha YCS1 Bonanza
1981 Yamaha XT250H
1981 KZ440 LTD project bike
2013 Yamaha FZ6R