There's a trade-off in terms of engine tuning. It's kinda bizarre....
The easiest thing in the long term is to swap in a more powerful engine, complete with its own fuel/spark management system. Take the whole thing out of the donor car (perhaps a Volvo 400 series, or a Renault with a 1.8 16v or 2.0 lump?), and dump it into the 300. It seems like a lot of work, but when it's in, it'll work exactly the same as if it were in the donor car - (hopefully) reliably, with reasonable MPG, good emissions...etc...it'll benefit from the millions spent on making the donor car what it was.
The other way is to modify the stock (or donor) engine, but this requires a great deal more skill in the long term, even though initially you're just talking about bolt-ons. There are ways around all of this, but none of it is simple, especially when there's next to no aftermarket support - this is no Civic!
Finally....exhausts and air filters might sound good, but they make next to no difference in terms of power (apart from in certain circumstances). There's a lot of marketting BS goes into selling 'neat' to install products to the Max Power crowd, but we like to go a bit faster than that

...but sadly, that usually isn't so 'neat'.
For drifting, I'd suggest you look at upping the power a bit later. A guy on Turbobricks posted (something like) this about how to make a good track car...
1. Brakes
2. Driver
3. Suspension
4. Power
cheers
James