Pretty much the only option to change your gearing is to go get a 36 rear sprocket. A 20 tooth front will fit with a little grinding and a 32 is about as low as you can go in the rear. Neither of those were actually made though so custom sprockets is the only way.
To do the porting and to open motor to accept the larger cylinder jug "the right way" requires you to split the cases and remove all the transmission parts. You need to remove a lot of material from the case, not only the ports and the diameter of the opening but also some parts in the very back of the case. The skirt of the piston dose not clear the small bumps in the very back of the port area due to it's larger diameter.
I am in the process of building a Metra80 2 speed and just put it all back together last night. With the correct tools it is a simple job. Two other people in SF are able to split the cases and remove the transmission with pneumatic impact guns.
Wail you are in the case make sure you open up the Main shaft and examine the dampeners, these can degrade over time and be quickly destroyed by a kitted bike. Your bike should have the newer black dampeners and are probably in good condition.