The kit you have indicated/linked to is the 4-stroke kit from GRUBEE and is probably one of the best of the 4-stroke frame mount kits available when the bicycle-engines.com folks have them in stock. The motor is a chinese copy of the Honda GXH50 and members here have been having pretty good service from them it seems. The reduction gearbox is the better of the two types currently available but if there is a weak link to these kits it seems to be the gear box. Mostly because of noise due to the straight cut gears. They also make the kit available without the engine if you want to actually purchase a real Honda motor as several members here have. Another good kit in the $400.00 range is the Titan from thatsdax.com but it is a rack mount rather than frame mount. Dax has just recently announced the he is out of stock on these kits and won't be shipping again for 10 weeks. If rack mounts aren't a problem The folks at Golden Eagle Bike Engines (see link to the left) offer the Subaru/Robin 4-stroke engines in 25cc & 35cc kits that are said to be some of the most reliable kits available and the Subaru motors ar said to offer 200 mpg performance.
ocscully