You would have to swap out the engine entireley and have some fabrication and welding skills. NOT A PROJECT FOR THE AVERAGE JOE. I recommend a suzuki quadzilla rebuilt 500 engine. THAT THING WOULD REALLY SCREAM !!
Considering a yfz450 walked my yfz450x I’d get a yfz450 put a 15/36 rear sprocket will give you top speed and get an 08 with the oil mod done and can mod if not already done. Fuel injection is nice but a lot heavier and a lot more complicated taking out of an already running atv.. kind of pointless putting in a lawn mower as it’s not made for it and will Be a ***** in a half rerouting everything for the lawn mower. I’ve done it to mine with a. Cr125 and it was a pain just getting the mounts right. Your looking at a lot of money after buying a 450 depending if it’s perfect running condition and no probs later on.. lot of $$$$
They are the same.. the R is better because it doesn’t have an external sump pump for the oil and ready to race. Some people like yfz450 cause you can fill it up and not worry about being low on oil and it runs cooler. But if your putting it in a lawn mower get a 08 yfz450r ( no oil sump pump) it’s internal, and easier to mount.. if your set on. Blowing a lot of money. Might as well do it to an atv besides an lawn mower.. you’ll go a lot faster too with suspension work and big 3 it’s all about suspension man. But up to you if you have all the money to do all that fabricating and such lol