You can get a basic healthy increase without headwork or rods.
RC used to sell the 30-30 SUPER SAVER Claimed 30 % power increase and 30% decrease in fuel consumption. (IMO your results could vary)
Kit had; 10:1 cast 836cc pistons, RC240 cam (exchange), 4-1 black exhaust.
RC developed this kit to provide you with most effiecient motor possible. Bysustantially increasing the volumetric efficiency, we are able to give you a kit that will make more HP and produce more midrange torque. Permits use if hi-speed gearing without losing low-end or midrange performance.
I agree mostly with the above.. i am not saying that you will szee an increase below 2000 rpm, there are trade-offs to increasing power in other areas. ( if I dont mention it, it willget brought up..)
The above is really good bang for the buck, dont skimp on compression, and add a good valve job, and lighten the alternator.
FWIW on a rolling start, my buddies early ducati Monster was not pulling away on my tired 836, thought the torquey twim would have got a big jump on mew at lower rpms.