Try the folks at MaxTorque
Http://www.maxtorque.com I bought one of there HTD clutches for my project that uses the Honda Motor. Their clutches are not cheap but they are a good value. The clutches thay offer are all oversized compared to the 76/78mm ones offered with the kits. Max Torque also has alot of experience setting up a special clutch for the Honda GXH50/Clone motors. Due to the short output shaft these motors need a special adapter bushinf/collar and also due to their idle speed and high rpm output for a 4-stroke they need a special engagment spring. MaxTorque has all of this already worked out. The Special Honda Clutches sell for $88.00 so with shipping, figure $100.00. The other thing you need to consider is inboard or outboard mounting of the driver. The photos below show inboard mounting of the HTD clutch. Also in the first photo you can see the custom bushing/collar. Max Torque is a great company to deal with and when you contact them by either e-mail or phone you are usually dealing with the owner of the company, a man named Jim Donavan.
ocscully