I'm going to be in the same boat as you pretty soon. I'm doing a semi-custom Foxi build for my girlfriend and want to do a few mods to the engine so it gets her around 36-38. I've got two cylinders, one off a complete engine that had the entire clutch removed. The other was an ebay special. Both 5041A engines.
Maitland pretty much convinced me that any large kit on that engine will blow the clutch up. I guess it's not very well made. Otherwise, I've taken the heads off and it's true what people say - porting a 504 is going to be very hard. I think the best way is going to be modding a custom intake from a Puch 14mm intake. I would also maybe drive without the air filter and see what it does.
I know, man...I really wanted that big 80cc reed kit that treats has right now. I'm sure it BLASTS. But from the sounds of it, the clutch won't make more than 100 miles.