The Cag is the flat minimoto engine with a reed valve on the back/bottom end of the cases. Several dealers will sell you a complete engine and reduction box: http://www.petrolscooter.co.uk/catal.../category/254/
Seddon somewhere in Ecky Thump land will sell you this if it suits: http://www.seddondirect.co.uk/produc...?productID=370
There are minimoto/petrol scooter places that will sell you various reduction drives that are likely to fit, including a variomatic transmission:http://www.petrolscooter.co.uk/gearbox-dr49-type-2.html
I don't know that there are that many bike engine kits fitted over here, never mind 4 strokes second hand.