Here's something I've been thinking of trying. It's a 2001 RM 250 seat. The lines look about right to me. Imagine a sissy bar for the rear mounting point,
The lines look ok but it just seems too big to me. The rest of the bike is just too small for that seat. On the other hand, if you like it, run it! Who really cares what I think about YOUR bike, just enjoy it!
I think it looks good, and you can make it even comfier with a sissy bar with the shocks on it. Now you can ride with a passenger, girls love a rumbly bike, rev it up and she'll giggle with joy, lol!
It's too wide at the back for reverse mounting scotto but I see where you're going with that. I think it will work best if I match the bottom line of the front of the seat to the frame toptube angle. A pedestal for a mounting point under the front and the rear mounted to a bar coming up from the stays and about an inch of clearance under the seat keeps all the lines right.
If you like it, fuh what others think! I think it will tie together well with some new covering stretched over it and a sissy bar. The 2 liter peanut tank looks a little small compared to the seat and kinda throws off the proportions, but a bigger one is always better and would fit right in with that seat.