If the water supply is limited, then you need be able to flush with a small amount of water, not free flowing water.
Muffs, especially if they are custom fitted to your specific
engine *might* be OK, (becasue I've never seen 'standard' muffs that did not leak profusely) but the feed to the muffs would need to be from, say, a deckwash
pump picking up from a bucket of fresh water (perhaps half full, so around 4-5L) with the return line from the telltale going back into the same bucket.
This probably requires getting a machinist to tap a thread into the telltale hole, so you can screw in a fitting attached to a hose to return the telltale outflow to the water bucket.
This might be tricky for 'main engines' that are on drop down mounts or whatever, but feasible for the smaller, more readily detachable
dinghy donk.
I recently watched "Wildlings Sailing" YT channel flush their small ancient 2-stroke dinghy donk with fresh water, just by sticking a bucket full of water over the prop and leg (i.e. they dropped the
motor into the bucket then fixed it to the rail so it wouldn't move).
They added some 'flushing solution' that was a purpose designed anti-scale solution to assist with the
cleaning, and the water definitely went darker and murkier during the 5 minutes they ran it. IIRC their telltale just dropped into the bucket...
But the motor then ran cleanly and pumped water again, which it had not been doing before the flushing operation, so they proved they didn't need a new impellor.
So a little bit of faffing around to save a quid or two.
But...it *can* be done if you want to badly enough, and does seem to be efficacious, at least, it was in their case.