There's a UK based MKIII owners site where you'll find more information on that rare 2.0L I6.... They're not a very desirable power plant and your resources will be quite limited outside of the Japanese market. I think this site has had less than half a dozen 1G owners and I don't think any have been active in at least a year.
As for upgrades, beyond a rebuild you're looking at the usual fare: fuel controller, larger injectors, boost controller and upgraded turbos... But I can't personally give you any specific recommendations for the 1G.
www.mkiiisupra.net might prove to be a good resource.
It is neat in its rarity, you'd be amazed how many people don't know that there was a 2 liter Supra. Good luck with it and if you do keep it I hope you will post more about what you do with it.