I've got an 84ish 560 Sonic that I bought many years ago, and by the time it had been thoroughly gone through by a competition shop and I got it home I found that I 1) didn't want to ride something that fast in the dirt and 2) I couldn't figure out where I'd hang the milk crate on the bike so I could get my foot high enough off the ground to kick-start it out on the track.
The bike came to me with an early 90s KTM disc front end with USD WP forks (I think damper rod type, not cartridge). I've got a NOS frame/WP rear damper/swing arm on it, and have the spare original bits too.
I need to get serious about finding a new home for it and I'm wondering if it is worth the effort to round up a different drum-brake/fork to make it legal for PVMX here in the States. If it can't make the grade for competition I'd just leave the disc front on and offer it as a big air-cooled play bike.
There's no concern about it having plenty of power, as the engine is fresh, it has been ported, custom pipe, big custom carbon/Kevlar airbox, new 40mm Dell'Orto, new Megacycle cam, piston, /rod kit, clutch basket, etc by someone experienced with the Rotax singles. If something needed fixing, it got fixed. I only rode it briefly/gently one time so I don't know if the bike would handle well enough for someone to be interested in racing it.
If it is worth the effort to swap front ends, what (available/affordable) period fork would be good to look for? I see no need to try and find the original Marzocchi if an RM/YZ/CR etc drum brake and fork would be easier to locate and purchase and also be an improvement.
Here's a couple of photos so you can see what I'm working with:
ETA: the Sonic would race in these AHRMA classes:
Ultima 500:
For certain 325-580cc motorcycles produced within the guidelines of the
Ultima Class specifications. Eligible machines include but are not limited to:
1982-83 Can-Am Sonic with Rotax four-stroke engine
1983-84 Husqvarna 510 four-strokes
1981-83 Honda CR450-480, 1981-82 XR500. 1983 XR500 is prohibited.
1980-81 Kawasaki KX420
1982-84 Maico and/or M-Star, air-cooled drum brake only.
1983-84 Yamaha TT 600 (not XT600). (Note: This is the only model allowed in the class
that is over the displacement limit of 580cc. Typical overbore guidelines still apply to
this model.)
Ultima Four-stroke:
Certain 200cc-580cc four-stroke motorcycles that were produced
with either a single-shock rear suspension or a newer engine technology. Eligible
motorcycles and modifications include:
1983-85 Husqvarna TC , TE, TX 510 four-stroke
1982-84 Can-Am Sonic with Rotax engine
1981-82 Honda XR, 200, 250, 500 with Pro-Link rear suspension (1983 XR 500 is prohibited).
1983 model XR350 with RFVC is allowed. XL350 models are prohibited.
1983-84 Yamaha TT 600 (Note: XT600 is prohibited). This is the only model allowed
that is over 580cc. Maximum overboer +0.080.
The USD WP fork is eligible for Ultima but I've not looked closely to see how easy it would be to hook up a drum instead of a caliper. I also don't know if someone would snivel with it still having the caliper mount in place. I suppose I should check that.
cheers,
Michael