On an old road bike the freewheel mechanism sometimes gets stuck in the "coasting" mode, i.e., the pedals spin but the bike doesn't accelerate. When spinning the cassette by hand it still works as expected, however with more friction than I would expect. So I am assuming the cassette needs to be removed, the freewheel mechanism cleaned and oiled an put together. I tried opening the lock ring of the cassette with a Parktool fr-5.3 tool which does not fit (the tool is too small in its outside diameter and the number of splines doesn't match). I don't find any tool online for 10 splines, could any one tell me where to find the right tool and what it is called? I am attaching a picture of the cassette / lock ring.
I have no idea what the correct Suntour tool is for this. This design only was made for a brief window.
One plan is to make a 2-notch tool out of a beater socket with the appropriate OD. Carefully grind it without letting it get too hot; even though you're only using it twice, you might have to really reef on it for removal. Grind wrench flats on the outside so you can hold it on with the QR while turning the lockring with a big adjustable wrench.
I believe I've also seen this flavor (30mm) of BMX 2-prong FW remove ground down for this need. They're cheap.
Ultimately you have the decision whether any of this is worth it, since when it eventually wears out, replacements are vanishingly scarce and also not really compatible with available indexing shifters/RDs. Unless you have a lot more time than money it's probably better to cut bait.
I would go with vise grips, plumber's pliers, or a grinder and remove it destructively.
Then you replace the whole cassette with a more modern one, which will have a standard modern lock ring.
Cassette teeth are a wear item so it needs replacing eventually.
The risk here is whether the freehub's spline pattern matches modern cassette's splines. If not, you may need to replace the freehub, or the whole hub, or possibly the entire back wheel.
Another option is to find an old socket whose outer diameter fits the larger of the two bore diameters, then grind, file, or mill ten slots for the teeth. This might be a one-time tool, so there's little point making it pretty.
The choice comes down to whether you will save this cassette and re-use it later, and the cost of a new cassette/chain vs the cost of your time and tools.