Hi and welcome,
As usual, money will be the deciding factor here.
Before doing anything to modify it you need to accertain the overall condition of the engine. A compression test carried out when cold and then when hot are a good starting point.
Drain the oil and check that there's no nasty crap in it. Same with the coolant, any oil contamination? Is the headgasket leaking at the corner above the alternator?
If the engine is mechanically quiet with no top end rattling etc and everything seems in order, the first thing is to give the engine a thorough service, not just an oil and filter change.
If the charger hasn't been serviced recently, get it done. For more performance, have it flowed and ported. You can fit a smaller pulley, better still a toothed belt conversion.
Then you are looking at a proper cold air feed to a heat shielded cone filter, a decent 4 branch manifold and decatted system. An oil cooler is a worthwhile investment too. With a remap and possibly some 250cc injectors fitted to a GT inlet manifold, that's pretty much as far as you can go without opening the engine up for a ported cylinder head and change of camshaft.
Obviously you can go much further. The bottom end on these engines are very strong indeed, the pistons being the only weak point. Forged pistons in a rebuilt bottom end will give far more long term reliability and allow even more power to be safely extracted.
150-160bhp is reasonably easy and relatively cheap to get. Beyond this it starts getting expensive - very expensive. Good fun though!
Hope this helps,
Cheers,
Richard.
