Another approach - try what you've go. Change the chainring. Shift into small-small. Does that work? I'm guessing the derailleur cage will be roughly straight back and unable to get full tenison of the chain. The chain may be hitting itself. But neither of these is a deal breaker if it doesn't bother the matter between your ears. It is a gear that you shouldn't be using very much. Working but meeting nobody's idea of kosher is OK.
If it doesn't, there may be other easy options. First, if you cannot make that derailleur work, it will probably be because it is a medium cage, not a long cage. SunTour cages could be swapped. I'm sure there are tutorials on line and if not, we can help you. I used to tour and race on the same Cyclone, just different cages. Cyclone - a lot lighter and sexier than the 7 and not interchangeable but no different in concept. You may be able to go to a coop and find a 7 with a longer cage. (It can be crashed and have a broken upper body. I used to rebuild my Cyclones after crashes and wear by buying crashed derailleurs from bike shop wrecked parts bins for $5 with the half I needed intact. Kept a Cyclone going for about 20 years and an absurd amount of miles doing that.
Ben