>>Rejetting the Carb: Speed Gain?<<
Unlikely unless you increase airflow.
>>Changing the sprocket: Speed Gain?<<
Yes, very minor..can go for power takeoff at the cost of top speed, or top speed at the cost of takeoff power.
>>.Biturbo Exhaust: Speedo Gain? Do I need to rejet the carb or do anything else to make this work well?<<
Very much gain, and it does tend to lean out the mix so jetting richer is reccommended.
Combined with the sprocket, it should get you that 40Mph you're hoping for.
>>Modifying the Air filter: Speed gain? My air box is 'permanently' sealed up..so I don't even know the internal construction. Can you clean them out? Do they need to be?<<
Best ask someone else on that, imma Tomos expert, mostly...tho I like Puch too.
Never did an airbox mod on one, however.
Not reccommended, especially for beginners...ask around, you'll see.
>>1986 Puch Maxi Sport 1.5 HP<<
Now, I don't recall offhand how it is done, but I know some of the forum members do - but there's a "Conversion" procedure for changing the 1.5HP Puch models into 2.0HP models, which isn't all that hard, far as I understand it.
My suggestion would be to go with the BiTurbo pipe and a richer carb jet, and the sprocket, along with the 2.0HP conversion, realistically you'd get anywhere from 40-45mph out of it, and that's all you really dare to grab for on a ped, especially as a novice owner.