I don't care what anybody thinks... DO IT! Unless there is a physical reason why the engine won't work or fit properly in a Blaster frame, it would be a neat swap to see and you'd have something unique. Don't let everyone's dislike for 4-strokes put you off the idea.
That said, I have a few thoughts for you which you may/may not have considered. First off, you'll need an extened swingarm to cope with all that torque, or else you'll be staring at the sky everytime you punch the throttle. Also, a Rappy engine is very tall, and I don't know if it would fit up under the Blaster's stock fuel tank. You'd have to measure and go from there, or possibly re-locate the fuel tank like lots of the streetbike/Banshee swap guys do.
Plus, can you fit a radiator in-between the front of the Rappy engine and the Blaster's frame? You'll have to get creative and find out. Furthermore, that is a heavy engine, so some suspension (and brake, if you're swapping it into a pre-03 frame) upgrades would need to be in order. You have to be able to control and stop the beast somehow!
And, lastly, is the end result worth the amount of work required to do this swap successfully? I don't know; its your project!

But just remember: I was able to beat a stock 660 Raptor with my Blaster's ported, but non-stroked, engine. It wouldn't take much more work than what I have done to a Blaster engine to make it powerful enough to give a piped/jetted/modded 4-poker a run for its money. After all's said and done, you might want to build a 4mil, ported 240 Blaster engine and keep it 2-stroke, yet just as fast.
Have fun, whatever you do! I'd like to see the results if you go through with it... I:I