Not sure if this is any help freaky but years ago before ebay and the internet I needed a good pair of inners for my MX 250. Searched a few local wreckers and found a set of 34mm forks from a 650 yamaha. I'm not sure what model they were off but they were longer than MX forks and had disk brake lugs on lowers.
They were cheap enough so I bought them in the hope I could use them. Pulled them apart and found them very similar inside to the MX forks just longer. So cut about 3" off the bottom of the tubes and remachined the circlip groove and fitted the MX lowers.
Raced with them for about 10 years and only recently replaced the front end with XL 250 forks and wheel.
XS 650 fork tubes are available new from here
http://www.mikesxs.net/products-19.html#products but I don't know what length they are.
