I have five saws as you have, with 16" bars on two of them. 3005 000 4813 is the Stihl bar number, and I have/use their 61PMMC3-55 chains. I do have 7-tooth sprockets on these two. Your dealer should (?) have them in stock -- Stihl recommends the "life" of a sprocket is two chains' use, so these are replaceable items. I have seen them, too, on eBay if you do not have a local dealer. My own preference is for OEM Stihl vis this part, and colleagues also use Sten; perhaps just me, but I'm wary re the many Chinese import "after-market" parts. To change sprockets you need but remove the e-clip and washer, and the sprocket you have will lift right out. I HIGHLY suggest replacing or, at least, re-greasing needle bearing under it; and simply replace sprocket with the new one. You may need rotate it clock-wise by hand a turn or two so the oiler mechanism is engaged!
One thought which is NOT "rocket science" is these wee top handle saws do NOT have adjustable oilers! Hence, the smaller the chain loop (e.g., 12") gets more chain oil than, say, a 14" or... the 16". For LIGHT cutting -- e.g., small stuff, I've not have had any problems. BUT -- I'd sure keep an eye open vis there's enough bar oil if I was using the saw for, say, bucking logs. (For this I have MS 261 PRO, MS 391, etc.)
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