Scaling out this kit would be a bit much for even the most adhearent scale modeler.
This model was intended to give one the appearance of a Spitfire in the air while giving it the flight characteristics of a sport model. I'm not going to discourage you from trying some of the mods that you talked about, but think them out carefully.
I'd leave the the retact set up the way it is. If it is the first time working with retracts it will be easier.
As far as painting goes it is worth a try. Just remember to keep it light. Fiber glassing could get a bit heavy. You might consider using silkspan and dope. There are other techniques so if anyone else has a suggestion lets hear it.
There are quite a few books out there that discuss finishing options. Some of them are by Harry Higley. There are others and there worth checking out and purchasing for your work shop.
Good luck and have fun !!! [&:]