80cc is not always faster then a 70cc , i't's all about the porting of the cylinder..
if the 80cc kit is a cast iron cylinder then do not think to much of it..
for Morini gsm the Gilardoni 70cc kit is the best kit to use ,std from the factory , and for tuning purposes cause of de internal shapes of the ports ,and it's aluminium with 1 pistonring.
there is potential , but only by tuning the cylinder the engine could create 20 bhp , but not with the exhausts that are avaible for the morini gsm , for that you'll have to get another exhaust from another model to fit.
so it is possible but not so easy as a Am6 engine , or Derbi..
The metrakit cylinder for gsm is a cast iron cylinder with 2 pistonrings , and a very low timing , not suitable for high revving..