The turbo flange is a standard T3, so many other turbos will fit. This is probably the easiest way to go rather than mess with a turbo wheel upgrade. I don't think there really are any upgrades for the turbo, and even if there were, they'd likely be pricey, you'd need to tear the turbo part way down at least and likely want to look into balancing.
You can get a HX35 for a couple hundred bucks that will give you more boost. They're used on late 90's cummins motors, and are fairly common to find in good shape being removed for turbo upgrades.
The turbine inlet should bolt up, turbine exhaust can likely be adapted without much trouble and the other fittings should all fit without too much work.
That being said, you're biggest problem with upgrades will likely be the physical space to make it fit. on the motor....