remember.. a lot is allowed.. civillian and military, from scale 1/76 and up... 
so, you could buy a fujimi kit, and make one of the complementary cycles that come with some ( either a harley, BMW or Zundapp )
or, you can go wild and get the Revell Satte Literschussel ( German comic figure Werner's bike, 1.444 litre engine lol ) 
Or, I recall the Airfix Wallace and Gromit tricycle... which would do nicely aswell... 
The possibilities are endless 

Best rule though: have fun 
