IF you have all of the peripherals (ignition module, throttle cable with built-in position sensor, coil, DPM manifold, wiring harness, etc etc....), it is a piece of cake... If you don't, and you are replacing a standard 670 engine, you CAN use the standard stuff on the HO engine. I've heard people say again and again that it cannot be done, but I have done it.... twice.
Install the standard flywheel and stator on the HO engine. However, you MUST check the timing. The flywheels are indexed differently, and you can just barely get enough movement out of the pick-up coil to bring it close to spec. Use the rest of the standard ignition, carbs, etc..... and bolt it in. The ignition is really the only hang-up in this swap. The HO engine may require a step up in jetting, maybe only a size or two, if you are using the standard 40mm carbs. If you are lucky to have the 44's from the HO, and are not gonna use DPM, use the '99 MXz670 HO as a baseline for jetting, and work from there.
If you dig back a couple years, I replied to a post explaining quite a bit of this in detail.
