That's a popular swap for the "mini mod 90" class for Mx'rs. The aftermarket support for the blaster frame is MUCH better than suzuki LT80's and predator 90's so people swap in a CR85 (or yz, w/e) and rock on.
There are a few disadvantages.... One, the 85's put out downright cosmic amounts of power (for only having 85 cc's) but they diddly squat for torque. Basically, you'd have to rev it to the moon to get it to take off well on a blaster (think race launch)
Also, making motor mounts isn't rocket science as long as they're thick enough and welded well. The real "rocket science" part is getting the pipe to fit and work right. The best way is to design around a stock 85cc pipe. Sometimes that's not always possible....
Why did you come to the blaster forum to ask us if a honda cr engine will fit in an older cr frame? Maybe you should find a honda motorcycle forum and ask there.....