I think for general shooting, a fixed 4X or 6X is ok for the Mach 2. BUT, I use mine mostly for gophers, and I like more power, for the style of shooting I like best ( rested on top of fence post, scanning through grass looking for them, or parts of them ). a 3-9 or 4-12 is great, especially with A.O., for those ocassional close-in shots. I think it's best max range is around 125 yards or so, although on a calm day it will hit at greater range, but bullet performance seems to tucker out around that range.
The HMR is good for another 75 yards or so, in fact, when you check out a ballistic chart, it pretty well shows a 75 yard advantage. So for the HMR, for the style of shooting I do, a 4-12 is best, or a bit more on the upper end, and again, with A.O.
You are lucky to have a Sportster in mach 2, hard to find on the used gun market, as you probably are well aware of.
I did send my .17 Mach 2 back to the factory to have the HMR barrel installed, but this was back in the "old days", when it was quite a bit cheaper then it is now. I also had them do their trigger job on it. If I were to get another Mach 2 ( I keep looking ), and also wanted another HMR, I think that I would just buy another rifle, as the costs come very close. If you do get the new barrel fitted, consider getting the forend that is held on by the finger tightened knob, makes for easy in-field replacing, and mine do not change zero when swapping barrels. Both are great chamberings !!!