Biggest problem is making up a pipe.
If you can design and weld up a pipe you can probably do the rest of the work involved.
The second largest problem is gear ratio. MX bikes have close ratio gearboxes. No fun.
you will be forever stuck with "variations of 3rd", never low or high enough gears.
I suggest you fix your Blaster.
Brand new cylinder:
NEW! YAMAHA BLASTER 200 YFS200 CYLINDER PISTON GASKET TOP END KIT SET | eBay
Here is a thread about fixing a holed case with JB Weld, yup, JB Weld.
Do it right and it will hold for a long time. Been running that way for a couple years now.
Not just running, but SCREAMING with over double the hp of a stock Blaster.
Give you plenty of time to look for new cases or bottom end if you don't like this fix.
How about a crank?
WISECO BOTTOM END KIT 88-06 YFS200 BLASTER YFS 200 CRANKSHAFT CRANK YAMAHA 89 90 | eBay
Hard to beat that? Fixed right up for $402 and shipping. Oh, and $7 for the JB Weld.
Probably less than $500 will have you running again, with better gears than can ever be found for the MX motor.
As for power? A messed up pipe for the YZ250 motor will make less power than your Blaster motor.
Plus, for less than $100 you can easily get that Blaster 3/4s the power of the YZ250.
Look at my back threads to see how.
What gives me those opinions?
Read the threads below.