They did run xr 69s with gs 1000 engines when classic tt race first started about 2013, then I believe the cut off was 1886 but only up to 1000cc, they then decided that they wanted lots of rc30s and owo1s and such things in the race so they made the cut off 1992 for up to 750s this of course would leave the 86 1000cc bikes in the stone age, so they decided that up to 31 december 86 bikes could go out to 1300cc to put them on a level playing field with the later 750s that handle better and should have a better power to weight ratio. I don't think there is anything wrong with this formula but I do have a vested interest as i run a standard framed gsxr1100 in this class and I must say we are never going to win.