Go with the 505. Or maybe a minarelli seeing as you'll be cutting up the mounts anyway. But the all depends what you want out of your ped? The 505 in stock form properly tuned is a very nice ride. The V1 min is a tad expensive but is a beast.
A 3rd option is pick up a 504 so it mounts right up. Then swap out the cylinder for either 505 one then toss on some basic mods. Tuned right yo,u should net about 35+ mph (or so I'm told). Basically high flow filter, up jet, bore the cylinder, and looooots of nitrous!!!!!! ...jk jk jk! Don't do that last part haha.
Browse through the wiki... Lots of articles on there that will definitely help you out. Also, just to confirm this ped style of yours post a pic of the mounts and pedal assy area.