Rods? You've got an oldfashioned pushrod version from the sixties? If yours has a 100cc engine it should be able to go abaut 70-75 Mph, if it's a 50cc engine it probably wont get past 40. I've got a tuned 4-geared 50cc engine with a SOHC and my bike goes 50 Mph.
Do I know where to get parts? I can get complete engines! If you need any parts you should surf to http://www.4-strokebikecentre.com/ and click on CONTACT. There you can mail to the owner of 4-Stroke Bike Centre. This shop deals exlusivly in 4-stroke honda's and has an international postorder service. The only problem you might have is that the site is in Dutch, but you'll be fine when you've found the contact-page.