It sounds like yourTY has been converted for trail use, Yamaha offered a large optional seat, I don't recall ever seeing one, how about a photo of your bike? A used Trials seat should be fairly easy to find.
The help for your suspension is easy to find in the last 40 years of motorcycle developement, as in a newer bike.
There is a Trial next month in Newman Lake, WA, about 46 miles from you.