That is the thing with TS engines - to gain more power is a very expensive task! It can be done but you have to be very commited
One of the guys I use for headwork gets almost 300bhp out of 2.0 TS engines and builds them for racing but this involves using a steel crank as well as many other super pricey bits! Nice but expensive!!
Best place to start is top end as it is less costly. Performance cams, ported and polished head, decent performance exhaust and you will see some small power gains.