"MAC" is a mispronunciation of the Cyrillic "MDS" - people think the "D" is an "A", and pronounce the "C" incorrectly (sounds like "S") and arrive at "MAC". It's pronounced MDS however. And they are strong runners, in the mid teens and a little above. The carbs can be leaky, some are good and some ain't (hey, I think I've heard that song before?). The fasteners on the older engines look like the screwdriver slots were cut freehand. But they look good internally. When in doubt drop a better carb on it.
Oddly, I was fiddling with engine bits a while ago and discovered that I can swap parts between an OPS Speed .65 and my FIRE MDS .61.. so out of curiosity I recently assembled a Frankenmotor - OPS .65 rear disc intake assembly, front crank housing, and crank, and MDS .61 crankcase, piston and liner. Fore-aft spacing, crankpin dia, etc are identical*. Why you say? No good reason, but since the MDS is timed for and has a reputation for pulling hard up in the 17-18k region, but has a crappy carb and known for breaking cranks if you push them hard, I thought it might make a perky piped sport motor for .40 size airframes. All I had to do was relieve the piston skirt just enough to clear the cutout on the rear rotor assembly. I started this a while ago but lost interest, and just finished it up a week or so ago. It's too bloody cold and miserable outside to run it right now, but when I do I will report these totally useless findings.
* if there is a betting pool as to who copied who, I'll put my money on MDS copying a known killer Italian engine. Can't be just coincidence can it?