I have one of the old aircooled jobbies, an ex enduro racer that has had a colourfull life.
In my experience of 2 strokes you realisticly have 2 options.
Leave it as is, tolerate 60mph and have a bike that will run to 20000 miles without any majour work. Which is superb for a 2 stroke.
Or make it faster, do the earth thing. OG or VEZ can detail that for you and fit a sports exhaust, the tailpipe really does very little except noise. A freer air filter, but keep the original airbox and richen the carb a little. All that may get you to 75ish. But you will be compromising the reliability of the bike, by how much depends on the myriad of other owners.
Something for you to think on, my TDR 250 has 4 times the power of your DT but realisticly tops out at around a ton. It weighs about the same and the aerodynamics are comparable to a DT125. A TZR 250, same engine, same power as my TDR. That can crack 140mph in perfect conditions. But, a TDR250 well ridden can lick a 90's 600 sports bike on a twisty road and totally ahnialate the TZR, its the same with the DT. Stay off the straight stuff, drag the racers onto the b roads and you will be king.
Something i used to do regular when i was using the DT as main transport when people gave it some stick i had a simple bet for them.
"Anywhere you like. You pick the way there, I pick the way back" Funnily enough never had any takers