I have been looking at old motors and some came up that awesome like the Maytag model 72D twin motor they made a ton there of the hit and miss design. If you geared it well you could produce enough engine bog so the engine doesn't stop firing then it would run well and a bit less vibrations and there kick start how cool 
there of 5/8 hp but thats true hp rated when they used this formula (lift 1000 lbs up 1 ft in 1 min equals 1 hp) this engine is about 2.3-2.8 hp now a days standards (i think) and is 45cc in each cylinder. With a bit of work you can make a different housing for the cylinders to sit in and make it a v-twin it has been done
YouTube - Maytag Bicycle‏
note in this vid the engine has not even got close to full potential
diff model Maytag engine 1 cylinder
YouTube - Maytag 92 Powered Bike The Agitator‏
there of 5/8 hp but thats true hp rated when they used this formula (lift 1000 lbs up 1 ft in 1 min equals 1 hp) this engine is about 2.3-2.8 hp now a days standards (i think) and is 45cc in each cylinder. With a bit of work you can make a different housing for the cylinders to sit in and make it a v-twin it has been done YouTube - Maytag Bicycle‏
note in this vid the engine has not even got close to full potential
diff model Maytag engine 1 cylinder
YouTube - Maytag 92 Powered Bike The Agitator‏