i agree with gscx...number one, yz250 engines are high dollar. i just had to buy a lower end for one of my yz's and the cheapest one i found (tranny, crank and case assembled) cost me 300 dollars. If you have a late 90's to early 2000's engine i wouldnt be wasting all that fabrication time it would take to install into a blaster frame..spend a few hundred bucks and get an older yz frame..if ya wanna know where to get one send me a PM...I have a few leads on a couple rolling frames for cheap. a bike frame is where that engine belongs. a yz engine is a f*cking POWER HOUSE , especialy in a light bike frame....
thats just my personal opinion, im an owner of both so im just speaking from personal experiance .
also dont forget that that yz engine is liqued cooled..so somewhere youll need room to mount a cooling system...that may be difficult. i bet my yz motor is atleast 4 inches taller then my blaster motor. from base to top of head...it has the coolent hose that also has to go into the head and that takes up room as well. that would be hard to push in a blaster frame.