You are best doing a turbo II swap. From what I have heard, the N/A rotaries do not take well to turbo. You also will want the turbo tranny and rear-end as well. I think the GTU comes with the LSD rear though.

The whole swap will usually run you $1500-$2500 depending on if you get the hood, rear-end, and the mileage. You also want to opt for the S5 swap.... 89-91 rx-7's are S5 models.