building hybrid 250 4 stroke yfz450 frame help please

[TSI2FLY1991]

Here is what I'm building is a 250 four stroke not sure what 250 engine to use my the class rules are 150cc 2 stroke and 300cc 4 stroke I had a yz 125 2 stroke in it but something wrong and overheats and have had it to many shops and no one can figure it out so I figured since I have some time to change set up I want to put a dirtbike 250 fourstroke engine in it just not sure what brand engine to use any help would be great and would you use dirtbike pipe or yzf450 pipe with 250 head pipe I know it will have to be customized just looking for some help thanks

[Logan #34's Dad]

Since your machine is a Yamaha, stick with the Yamaha motor. Find one from a YZ250F. You can put a big bore cylinder on it pretty cheap and the 2008 head is said so work best. The stock head already has Titanium Valves.........
ebay is a good place to look.. Your gonna want carb and electronics too. Might be just as easy to buy a complete bike and strip it out then sell the roller...

[TSI2FLY1991]

And this is why I wanted help no matter what engine so is 2008 only year head or 08 and newer if I decide to do crf what year does people,use are they all the same the only thing turned me away from yamaha is the external oil reservoir and one more thing to have in the way and mount but if its the best swap

[guy310]

I have everything you need to bolt in a kx250f including the mounts, pipe, engine, carb, all wiring, intake, etc. We built one and used it for a year and now my daughter is old enough for the 450. Let me know if you are interested.