I'm currently collecting parts to build something like an old style boardtracker or possibly something more street focused with mudguards/fenders (it gets wet here!)
So far I have the remains of a wrecked old Pashley delivery bike which will donate:
Complete compact rear triangle.
26" rear wheel with Sturmey Archer hub brake & 3 speed internal gears.
Brake levers & gear selector.
Front 'Sturmey Archer' hub with internal brake (on 24" rim so needs rebuilding on 26").
Possibly stem & bars.
Narrow gauge tubing for fork brace.
Possibly chainset, pedals & bottom bracket.
I've also rescued an old 22" Schwinn CrissCross frame which may donate it's front triangle, headset and possibly forks (may try to find some older flat crowned forks or springer forks instead though).
Originally I was just going to stretch a Schwinn Corvette but as I've got this lot it I think it makes sense to try & replicate a boardtrack frame.
Found this forum (and the 'Ratrod' one) when googling for a 4 stroke gas motor kit (will do a search/new thread for that as I'd like something with a long stroke if I can find one).
I need to find the best way to fabricate the lower loop in which to mount the motor too. UK plumbing pipe benders only go up to 22mm so I've thought about trying to find a childs bike with a single 'U' shaped frame to use for this section.
Don't know any other UK builders - gas bikes aren't legal on the roads here so it's either add lights etc and get it tested & approved as a sub 50 or 125cc custom motorcycle or just pedal it between unclassified roads.