Depends on how far you want to go and how rich you are feeling
A decent exhaust, (like the ones I sell, plug plug)
A carbs (twin CD175's work well), not hard to pick up a set. (I probably have a spare set about somewhere)
Head porting, bigger values etc. Any good engineering shop can do this at a cost.
Lighten and balance everything. Again a good engineering shop at more cost.
Hotter cams are hard to find but do appear from time to time.
The slant motors are very robust and reliable if built properly. The bottom end is the same as the 2300 so it's basically unbreakable in 1800 form.
Change the oil every 3-4k and don't use cheap stuff and you will see 100k between rebuilds.