There isn't much available in terms of parts for performance upgrades, KG Trimning
http://www.kgtrimning.com/defaultE.htm used to have a few different cams, headers, Weber manifolds etc. on their site, looked like good stuff but very expensive by the time you get it here.
It looks like Skandix also do a few performance bits for them:
(great old video, the current affairs shows would be all over it if it was Australian!)
Basic cleanup of the ports is worthwhile on most engines while it's apart, and careful smoothing of sharp edges in the ports isn't too hard to do.
I'm not sure on valvetrain upgrades, but anything designed for a B18/B20 will work as all the parts are the same.
People have been boring them out and fitting B21 pistons for many years, the displacement ends up being roughly 3.3L.
Fitting a set of steel timing gears would be a good idea, the original setup uses a steel gear on the crank and a fibre gear (to reduce noise) on the cam, a fully steel set is more durable of course.
