I love those GC engines, a lot easier to mount than making a vertical mount for for a GX engine. Start with the motor of your choice and draw/build around it. A big sheet of card or a piece of plywood to draw on, before you know it you're cutting tubes and putting a bike together.
Don't exceed 5.000rpm with a GC series engine. A valve or something will break through the head or cylinder of the engine. They are a very good looking engine with adequate performance. When used with a torque converter the GC160 will peak at right around 5.000rpm. However, torque converters aren't appropriate with board track style bikes.