Hmmm.... I don't know of to many people that modify gx120s, usually the smallest engine that we use are 160s. Hearing this, most of the after market part sites are for 160s and up. Sorry, but a majority of your parts will have to be custom if you want to reach the 6-8 hp range. The only things that I could think of are to remove the governor and choke and get a TAV-2 for a your drive system.
*Note- The removal of the governor will get you more rpms (speed) not any horsepower.