What's a ballpark budget to get my 13B running in a midget? I suppose I'd also have to upgrade the brakes and suspension.

 

I couldn't tell you. Mine had a 12A in it when I got it.

 

These links might help you.

http://www.geocities.com/mg12a/mg12a/intro.html

http://www.cool-screens.com/Rotary-MG_Home.html

http://hometown.aol.com/ursnoozin/myhomepage/auto.html

 

one of the most expensive parts is having an exhaust made if off the shelf parts won't fit. If you can make one, it shouldn't be too bad, but you need either racing beat's thick steel tubing or stainless. Other than that, the better the donor car, the easier the swap. You can reuse stock master cylinders for the clutch, just get a stainless line to mate to the mazda slave. Custom driveshaft can be had for around 100 dollars if you bring them the mazda and mg driveshafts with a length.
You really don't have to upgrade the suspension or brakes to get it going. Don't make the mistake too many make: Go ahead and put the engine in, get in running, then upgrade, rather than doing everything at once. This way you can DRIVE it while you upgrade the suspension and brakes rather than waiting for money, time, etc to do it all at once. I did this, and it causes much greater motivation since that way I could jump in it and roll most of the time. Too many people try to do it all at once, and either run out of money and never finish it or get frustrated after 2 years cause it still can't be driven. Good luck.