Man most people are going to tell you to put a bigger motor and GBox in, id say depends what you want.
As a nice original condition that has been in the family since new...
Rebuild the stocko, if it can be, and keep her a stock cruiser.
If the motor is beyond repair, then it depends what you want.
A nice stout 2.6 will be plenty in a 800kg car.
Upgrade front brakes and suspension and you have plenty of fun, add some webbers and she will be sexy under the bonnet too.
Or from what you are suggesting,
dont go the MX5 engine, shit and heavy, and very costly to get power out of them.
Rotary, you would be looking at non turbo, and even then depending on your state difficult and very costly to get legally in a car this light.
4G63T, great power, prob too much over kill, will be quite expensive to set up.
But at the end of the day, great value for money, and very tidy!
Depends on your budget and what you want from it???
Sticking with Mitsubishi is definitely easiest for Engineering.
And these guys will be able to help you out with a Sigma Block, either 2.0L or 2.6L and a matching Magna Head for Injection. And then you could look at a turbo set up.....
But big upgardes in Fuel, Brake, and suspension setups would be required.....
Figure out your budget, a plan of attack, and what you want out of it in the long run