Overall the motor sounds good at idle and during the light revs. The lack of torque is a fairly common complaint when these motors are stock and to some extent even when modified. It's the general nature of the 2 stroke beast, they rev higher/faster, typically have more hp/torque than the equivalent 4 stroke but the power comes on and leaves within a very narrow powerband (usually 6.5-7.5k rpm, but that is purely pipe and intake runner dependant). Whereas a 4 stroke will have lower peak numbers but that power is usable over most of its rev range.
The biggest "single bolt-on" improvement that can be made to these bikes is throwing on a decent expansion chamber. It's not unheard of to gain 1hp or more when putting on something like an MZ65, even the fairly craptastic zeda88/cdh66 expansion chamber should be good for about .5 hp. The downside to this is without an additional silencer the bike will be stupid loud.
For me the combo of an mz65 ($75 used) and a generic silencer ($20 - Amazon) made my bike feel like I strapped on a different engine altogether.
If you have a dremel (or equivalent) the best thing you can do for these motors is clean up the intake, exhaust and transfers of any casting junk that is left in from the factory. No need to grab a timing wheel or anything like that, just simply remove the flashing/sharp edges. Anything that would impede the smooth flow of air should be ground down and smoothed out with the exception of the surface roughness of the intake, leave that alone for the best flow of air/fuel into the motor. This alone can gain a 25-50% power bump over bone stock depending on the motor.
Matching the gaskets to the openings can also make a fairly large difference, it's not uncommon to have the intake or exhaust gasket partially block the opening.
Some people have noticed a nice increase in lowend torque using a longer intake runner (heater hose works beautifully for this) of 6-9in. This is a finicky process that requires tons of testing to determine what length works best on your setup. Basically, just find a bit of tube to clamp your carb to then clamp the hose to that and the intake. Start long and slowly trim it back until the torque increase is where you want it. The 2 stroke tuners handbook says that it should feel almost like when you "get on the pipe" with an expansion chamber but at a lower rpm, however, with these small motors, it will be far less obvious than that.
