I too have wrestled with a couple of these clean-out screws, mostly in the sides of snails. Really wished they had not been there, in the first place. Most become buggerd up and look like heck. The last one I repaired, I had to drill out, re-tap and replaced. I was fortunate that our local True Value hardware store not only had the hex heads, but the slotted as well and after cutting to the proper length, fit very well. ....
Whenever encountered on new M/L's, I recommend that a good anti-siez be applied to these threads as well. Not only for yourself but for the next guy. ....
On older ones, leave sleeping dogs lie !!! ....
Be Safe !!!