This is the 125 part of the form, but all the S&T motorcycles have a separate cylinder from the case and the head, so you can have most any machine shop boreout the cylinder, BUT some inf you should think about.
The cylinders have a cast in steel/cast iron sleeve that sticks out the bottom of the cylinder casting, this limited thr size of the boring you can do, unless you also bore out the case and make a new sleave.
Then we have the problem of pistons, oversize pistons in the size that will work with the existing rod and wrist pins will run around 400+$ for the minimum of 4 to have made.
From an engineering standpoint the existing crank and bearings are at the practical limit.
For a race only engine that you do not expect to last long, good luck, go for it.
Would be easier/cheaper to stick a Suzuke 500cc in the 125 frame
