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King Hauler upgrades- The limit depends upon your bank account
The 1 thing that is almost a must is a mechanical fix- you need to add some sort of gear reduction unit to slow the truck down to manageable speeds. Using a high turn motor and a smaller pinion will work but it leaves you with a lot less torque than a gear reduction unit. There are a few units out there that are reasonable in price. Integy is one of the best and normally it can be found for a nice price.
Hope this helps!