First mod i always do to my race cars, remove the fuel tank, get a 1 gallon alloy tank and mount it to the roll cage, you'll never need too much fuel on track, it just adds weight, plus the original tank is quite a heavy bugger!
Few other mods i would do is remove aircon, electric windows and replace those with acrylic sheets, remove carpet, dash, actually - remove all the interior, get some steel rims on there as they'll be a lot lighter than alloys, i'm not sure if Wellar wheels made a 98mm pcd but if they did, these are the best steel rims i've used for racing, As for the engine, it will be expensive but it depends on how big your wallet is and how much you love the GTV, if i had the money i would go for the 3.7 autodelta with supercharger,
If you're going to use a supercharger, go for a an Eaton M62 as it's built for 2.5 - 4.0 litre engines, you'll have to play about with where to install it but it won't be hard if you're going to do a lot of modding
As for the anti roll bar, i can't remember if you can get a larger one, but consider the suspension mods and a strut brace first before you touch it, sometimes you might mess the handling too much!
I like the idea of the crank, i did this on many occasions with Cooper 'S' mini's, it also resulted in a higher redline but not sure how the GTV will do with that to be honest!
Hope you get somewhere with this as there aren't too many racing GTV's at all!
You've just been done by a Twinny!