Well, I can say right now it will be a bit of a project. This is what I had planned when I first got my GT, but I was having problems getting a line on a motor and tranny. I do know of one guy that was working on the swap, had the rotary and the GT, but I haven't heard from him since he started dismantling things to swap over.
But here's the info I collected when I was looking at doing the swap...
Weight wise, the 13b weighs less than the 1.9L in the opel, it's also a shorter engine. As long as you keep the engine to the rear of the front axel centerline, you won't mess your weight ratio up. I never had a engine and tranny to figure out the length issues, but I think I had it figured out that you could get the 13b shifter to line up with the stock shifter hole if you redid it and moved it forward a couple inches. The way the shifter is designed, the input shaft can be shortened and a bracket made to move it forward.
The stock Opel rear end won't handle the power of the 13b though. I planned on getting around this with installing a ford 7.5" rear end out of a ranger pickup. This would give you the strength needed, plus lots of choices for gears, differentials, and brakes.
Exhaust isn't too big of a deal, take it to a good shop and they'll be able to put just about anything you want on it, then only thing you're going to run into is clearance issues if you try and place the muffler under the floorboards.
Mounting issues, you're going to have to design your own engine crossmember, no way around that. I didn't have the engine to line everything up and get it figured out, but it should just take a little sliding around to get it right, just make sure you keep the engine rear of the front wheels.
Are you going with a T-II engine? or just a normally aspirated? Are you planning on using the stock ECU, or some aftermarket like a Haltech? And are you planning on leaving any of the emissions stuff on the 13b? I think on the 88 they used the air pump to open the seccondary ports on the intake. Just a few things to keep in mind.
I wish you the best of luck on this, I've only seen pictures of one GT that completed the swap, it's a great idea, just wish I had the time and mony to go through with it. Keep us updated on how it goes.