Honda CBR300R | Engine Rebuild. 

Exactly what I was thinking

The engine of the CBR 250r and the CBR 300r are basically the same except of the slightly longer stroke of the CBR 300r which leads to better efficiency and a little more power. You would not notice any difference I guess.

yes you can use it, but its not only that, needs a cylinder block for 300CC because 300's crank is little bit longer than 250.