There are a number of bolt holes on the underside of the engine, so I assume one gets a steel/ally plate about 1/4" thick and then drill to suit. Probably after mounting the plate to the seat post and the rear frame tubes, in order to get the chain line right. Since I need to get the overall ratio down to about 30:1 for the 1kw classification, it's going to be fun finding a way of mounting a 70+ tooth sprocket to the back wheel.