If you want something cheap and its will make it more reliable, 2-3 things, first swap out bushing for bearings, if it has them, the teton should already have bearings out the box if you bought it within the past 2-3 years easiest way to check is to remove 1 wheel and remove the hex, is there is a metal bearing you're good, if theres a plastic bushing swap it out asap before it gets loose and causes diff issues.
Second thing would be a lipo and a lipo charger, i recommend the OVONIC 1500mah 2s lipo, they fit in the stock battery bay without modifications and you only need an adapter to use it on the Teton, again depending when you bought it you might need a deans-to-tamiya plug or a deans-to-traxxas adapter. Traxxas connectors are about an inch wide, black and soldered, tamiya connectors are white and are crimped. As for a lipo charger i reccomend a REEDY 324-S its a great charger I own 2 of them and you can change the amperage at which it charges to have a full battery in about an hour or less at 2 amps. Make sure if you buy this lipo, to use a LIPO specific charger, not LIFE or LI-ION, or LI-HV those are different types of batteries and if you use those modes or charger you could cause a 🔥. This applies to all batteries but lipos are not to mess around with, ive never had a fire before after doing these mistakes but it never hurts to be precautious.
Now once you have made both of these modifications you can go ahead and get a brushless system for your teton. There are a ton of brands out there from cheap to more expensive than the car. I personally run a cheap chinese one and its been perfect, and more than doubled the speed and torque of my rally. If you are going this route make sure you at least have some steel diffs to handle the power. For motor i reccomend a 2430 size motor, and a 25-35amp esc if you google these specs on amazon there arw a couple of options or you can buy them separately. If you get this motor try getting a 4000kv or less any higher and its rpm will be way to high and it will get hot if not geared down. I own a 5800kv and its usable but i need to have a fan on my motor to keep it cool during the summer. Depending on what esc you get it may or may not be waterproof i wouldnt risk it unless its a hobbywing or castle, both of which make very good waterproof electronics. Another benefit to this is that brushless motors dont have cooling vents like the stock one which means, dirt wont get in you spur gear through the cooling vents and both your motor and spur gear will last much longer.