All the Monza has had done to it is:-
1st power pipe, porting, upjet, modified airbox, & later pod filter
2nd variator.
3rd tyres.
Here are some of my methods of mean machine making. 
Firstly, get yourself a powerpipe. Nothing will happen with your standard pipe, for sure.
Then, I'd grind the crap out of the exhaust port to make it look like the picture below.
What you're doing is to grind out the crescent shaped cast-in restrictor & matching the diamiter to your
luuuurrrvely new exhaust.
You'll need to rejet your carbi.
Whatever, mods you do concerning the inlet & exhaust etc do "plug chops"......which means, at various
throttle positions kill the engine & make it stop asap to determine the state of the plug whether it's lean or rich etc.
Better to do plug chops with a new plug, is my preference.
You can do searches for more info on "how to" plug chops.
What I did with my 50cc racing Monza was to grind as above.
I got myself a 95 jet that I had spare, & kept drilling the airbox till the plug chop looked right. You can always
bung the holes up with silicone sealant if not needed.
Or if you can buy a few jets plus or minus sizes either way of your target jet size, & increase jet size till good.
With the airbox snorkel out & all those holes in it, it was pretty close to equalling a pod filter which is now
what is fitted although these are noisy, if that's a concern.
I'm running with old rings, but I highly recommend renewing those.
Generally make sure it's all de-coked & clean. Change the piston bearing while you're at it. I see it as cheap
insurance.
When I initially ground the exhaust port I only ground the outer side of the exhaust without touching the
barrel side of the exhaust.
What can also be done is to grind the barrel side of the exhaust port to an increased height.
I'll show a series of standard portmaps & one tuned one I found on www which are simply tracings or
drawings to show the layout of the ports in the cylinder. 1st one is my old standard Bandit portmap.
It's a shame the MHR 50 doesn't show the height of the exhaust port, but you can see as the maps
progress through to that TP Trophy, & finally to DarkDream's modded cylinder that the exhaust port
is all about getting the gas out quicker & earlier.
You can see the MHR 50 has those auxilliary exhaust ports......we can't have those in our std
cylinder, but we can raise the height of the port similar to Darkdream's example.
Which is what I did to the Monza.
There are many 2-stroke tuning books out there, & normally port timings are expressed differently
than is shown, using a degree wheel.
I'm not inclined yet to go down this path, but I've had good results from that simple grinding.
Important to very slightly smooth the edges of all your ports edges in the barrel.
Here is a better view showing how the top of my grinding in the barrel is a nice arc & the bottom of the port is the
original & unground profile.
I didn't bother with polishing. Was just concerned with overall consistency & profile.
We've had a lot of fun with the Monza 50cc Racer.
I have the luxury of old back-up cylinders if my grinding didn't work out right. Fortunately, seems all good. 
