Admmmm
Just a thought, have a look at the Greenwood mini, in the 1960's ? Greenwood built a three wheel mini which he raced against sidecar bikes (even by then integrated platforms) this thing with a Cooper S motor up front blew the pants of the bikes, the Cooper engine was not all that powerful but it upset the sidecar boys.
In order to get the price down you need to use volume for the supply of units, a 3 cyl engine is very smooth, as a lapsed biker the BMW K75 750 3 cyl was very smooth but lacked grunt, the 4 cyl bikes could be quite rough, so try the beefy 3 cyl Triumph DOHC motor cycle unit, it is water cooled and can fit together with a radiator/Morgan grill also the arrangement allows shaft drive so the engineering is reduced, yes I like the Vee twin but it will need a lot of engineering because Ducati have a 90 degree vee and the forward cyl is over centre whilst the rear more upright, if looking for a vee twin the Moto Guzzi Griso motor at 1100 cc should do the job this too has an inline gearbox and shaft drive. Traditionally Guzzis are air cooled this is too.
Good luck
Christopher
