RE: MDS .18
The engine itself is not too bad for a ballbearing, true ABC, engine and without the muffler it weights: 172g (about 6.05 oz)
The stock muffler is a quiet one, with an internal baffle, and it is made very bulky. It does make the engine very quiet at low revs but the weight is a little high. There are also two versions of the mufflers, the older one that came with the .17 and early .18 engines weights about 45g (1.6 oz) while the newer one weights a wopping 60g (2.1 oz). The older muffler has an engraved "MDS" marking while the newer (and heavier) has a casted "MDS" text in the same place (near the inlet).
The bolt pattern is the same as most other .15 engines though, and the stock muffler (without baffle) from the Magnum/ASP .15XLS is a direct drop in. It is much louder though and doesn't improve the power much on small props, but the weight of that one is only about 33g (1.2 oz).
So with the Magnum muffler one would get a total weight around 205g (7.2 oz) which is quite competitive.
One could also rotate the cylinder (before the first run) to get a rear exhaust and put on a tuned pipe, but I haven't tried that (yet).