Try RD350 stuff.
18" rims front and back, disc on the front, drum on the back. If you feel the need for a bigger disc, the early 650 disc has the same bolt pitch and offset, but you will have to relocate the caliper (fairly easy to do). One of my RD's has this lashup (I'm using Betor forks). On a light bike, it will pull your eyeballs out when you clamp on the binders.
If you want to get really old school, try 1970-72 R5 Yamaha 350 parts- Double Leading Shoe front brake, not disc, and drum rear brake.
Probably some of the reallllllly old Yamaha 250-350 stuff from the sixties would fit- YDS3 and DS-6 (250's) and YR-1, 2 or 3 (350's). The 350's were faster than your 400- mine was clocked at 110 MPH true speed one time and I got it stopped (but that is another story), so the brakes are up to the task.
The issues you will have are maybe a spacer for the sprockets and up front, probably an axle spacer plus a brake stay from the fork leg to the backing plate. Drill the snot out of the hub and backing plate and rivet on a big airscoop for cooling and it will look bitchin' and help it keep cool. Those old drum brakes could get really hot. For reasons I can't remember after all these (35+) years, one time I reached through the spokes to touch the fins on the drum after a particularly "good" ride- and burned 3 fingers...badly.
There was also a really econo XS400 built with wire wheels and drum brakes front and back, but I don't know if they were ever imported to Canada. I saw them in the USofA and the UK and their wheels would probably fit with the least modification.
Or go crazy and find some old TZ250 Yamaha parts with alloy rims and huge brakes.
Hope this helps.
Have fun.
Pat Cowan,
Pacomotorstuff