Everything depends on your goals. You wouldn't do the same mods to a race bike as a daily commuter. Since we don't know your goal, frame, or current parts I have some general info that may help.
First off, don't forget supporting mods first if you wanna go fast. Stock department store bike wheels and brakes won't last long on these things and can be dangerous. I thought I could get away with stock wheels on my test bike since it only gets ridden literally 3-5 miles a week or so, and the rear wheel hub has already loosened up twice on me and once time caused the bicycle chain to jump off when applying the brake at 45mph. Luckily I have hand brake on the front with good pads so I was still able to stop, but can you imagine rolling towards the main road of your neighborhood at over 40mph with no brakes if I didnt?!?
If you plan on riding a lot, I recommend the newer sleeved G5 cylinders and malossi circlips. I use the single end malossi ones from treatland, they don't take any modification to the piston to install and are much better quality metal than the stockers that tend to bend and don't spring back. You can hear them snap into the retaining groove nice and firm.
The chrome on the normal kit cylinders has been hit or miss lately, and some are coming with bad/thin chrome jobs that wears away within 500 miles. Others have chips and marks right out of the box. A G5 sleeved cylinder is only $15 more than a normal one, and doesn't have that issue and will last much longer at the expense of being more difficult to port due to the divider in the transfers that should be removed for high RPM use. Will still do 40mph out of the box with the dividers in place though. Larger intake and exhaust ports which is nice if you don't plan on any port work but I still recommend at least cleaning up the sleeve/port alignment.
If you just want your average ~45mph build on a budget and nothing crazy, just matching the transfer entry with the case and some clean up to match the sleeve(the sleeve ports don't always line up perfectly with the jug) is all ya need. Throw in a good plug and play bolt-on pipe like the mz65(or cdh/zeda66 if you can't find one) and you will easily hit 45mph assuming your jetting is correct - if stock carb and near sea level, usually a 68 jet is good but start rich and work your way down till it no longer 4strokes/sputters at the top end.
For a toy that you want to focus on speed and not necessarily driveability and reliability a VM18 carb with an OZ reed, window piston, and dirt bike pipe on a ported jug will do 55mph easily with mild portwork and a 32t rear sprocket. For the pipe the kx85, CR80, or YZ85 are the biggest and best for top end, KTM65 and RM80 are a little smaller so hit earlier in the RPMs, but there are tons of applicable 2 stroke pipes out there to convert. I have pics of the KX85, YZ85, and RM80 to compare since I convert pipes on the side for my builds or to just sell outright.
