Well as an ex-sw20 owner I'll contribute what I can.
Since it's a 91, I'm gonna assume you have no LSD. If you have adjustable suspension, set them to full stiff in front and full soft in back. This will help tame the rear end and the snap oversteer a bit. If you have some money go ahead and purchase a front sway bar.
Without an LSD you are going to spin, A LOT! Don't expect to pitch the car sideways really fast the way you see S13/S14s do. Easiest way to drift in the MR2 was under braking. You'll need a little speed to do this. Turn in and lift off the gas or lightly use the brakes. The tail end will start to come out. Be ready to countersteer VERY FAST. In fact, easiest way to countersteer in the SW20 is to just ease your grip on the steering wheel. Once the car starts sliding sideways, you can feather the gas to keep it going. You'll be spinning out a lot when you start, so I wouldn't recommend trying this on the street.
Hope this helps. Good luck. Hope I see more mr2s at the events. I love those cars
