I told him to look into ditching the SLA batteries and upgrading the controller if possible. I don't know if getting more power out of the motor those things come with is possible
Definitely go with a lithium pack. If the motor is brushed, it can likely take 1/3rd more voltage without any significant increase in the wear-rate of the brushes [due to arc-ing]. If it is 36V, go to 48V...if it is 48V, go to 60V / 72V. Controllers get somewhat pricey-er above 72V. If the motor is brushless, you have many more options.
As far as the amps go, add a temp sensor and a motor-fan...and slowly raise the amps until its still below 93C / 200F as a temporary peak of less than five seconds [during acceleration].