That is the exact samr box that comes with the BGF kit cet the BGF one has a longer output shaft to keep the engine centered between the pedals.This gearbox will fit your motor.
only $34.95 with shipping already included !!!
Gas Scooter Mini Moto Bike Parts Transmission Moped: eBay Motors (item 180360796487 end time Sep-16-10 01:06:19 PDT)
It has a single row #25 chain. I don;t expect it to last long woth chainlube or grease, so I modified mine to run woth oil bath.
The box that DAX sells is built with stronger casing and uses double row chain.
I thinjk the g-4 is better than the others, for longer reliable service. That,s the box I'll get if my chainbox wears out too soon