I have the Blaster swap. Go with a four stroke, it will cost you about the same amount of cash and in the end you will be happier.
How fast do the YSR/Blasters go? They have just a little more top speed than a Derbi swap with an SP72 jug but a crap load of torque that the Derbi doesn't. This is with a bone stock Blaster motor with a stock carb and an RZ350 OEM pipe. I'm sure a small porting job and a 30mm carb will make the blaster swap faster than stink, but I would still rather have a four stroke.
Here's a vid of my YSR topping out. It has gone a little faster than that, but it was windy as hell and I couln't tuck in cuz if I did the cam wouldn't have captured the speedo.