Re: Can I use an old lawnmower engine?
Welcome to the forum. Check with your licensing people but you are going to find that the bike won't be legal. Thank the Euro people for that. I believe you are only allowed electric bikes and not more than 250w and if you don't pedal with the bike when the motor is on it won't work.
Look up page 16 in the Board Tracker section here on the forum. Towards the bottom you will see the thread, "Villiers 98cc BTR is finished" by Harry76. He built a super nice board track bike with a Villiers 98cc mk1 lawn mower motor.
Also my mate Silverbear is building a bike powered by the same engine. He will be back at it this summer to finish it up.