All Seeley frames have 27 degree rake. The trail is determined by yoke offset and wheel size. With 18 inch wheels and about 30mm of yoke offset and a wheelbase of 1454mm, you can brake before the corners and power through them - the bike oversteers instead of running wide through understeer. With 70mm of offset, the bike can stand up as you brake and throw you off-balance. ( Those are the two extremes ). The trail with 30mm of offset is about 102 mm. A lot depends on where the centre of gravity ends up. My motor is as far forward as possible. The rate of self steering can be controlled by setting the adjusters on the rear shocks.
If I was doing what you want to, I would borrow a frame and copy it. I would not use isolastics.