Just realized the typo. I believe it is an LM317HV, but I'll double check tomorrow. Here is a link to the amazon page.
https://www.amazon.com/dp/B081NJ12M3/ref=cm_sw_r_cp_apa_i_o7N8DbA6SY6NA
Sorry for the inactivity. I've been a bit busy.

 

Amazon page information -->
Title:
PWM Motor Speed Controller 9V 12V 18V 24V 36V 48V DC 9V - 55V Pulse
Width Modulation Regulator 40A 2000W Stepless Variable Speed/Forward
and Reverse Switch Pulse Width Modulation DC Speed Regulation… $19.98

top description:
connector-type Connector_type
Brand SP
Warranty Type Lifetime
Number of Ports 4
Current rating 2 Amps
About this item
Input voltage: DC 9-55V.Output voltage: Load Linear.
Input Current: require large than the motor operating current.
Output Current: determine load size.
Continuous current: 30A within.
Maximum power: 2000W.
Quiescent Current: 0.01ma.
Speed type: modulated current.
Speed mode: ON \ OFF linear potentiometer 270 degrees.
Speed range: 0-100%.
Operating frequency: 25KHZ.
Connection Type: Barrier Terminal Blocks.

Product description

Features:
1. Wide range of operating voltage: 12V, 24V, 36V, 60V, max. power: 400W.
2. A good speed controller for DC brush motors.
3. High-frequency capacitance, power tube and 10A fuse are used in the module.
4. Adopts sealed and high-quality potentiometer with switch, stable
performance.
5. Metal case creates better durability; high efficiency and low noise.
6. Suitable for applications like fans, etc. providing nice
adjustment of motor speed.

Specifications: Operating Voltage: DC 12V/24V/36V/60V
Rated Current: 8A
Max. Current: 10A
Control Power: Max. 400W
Speed Control Range: 10% -100%
PWM Frequency: 21KHz
Item Size: approx.70*40*30mm/2.75*1.57*1.18''
Item Weight: approx.73g

 

What a mash of specifications in the amazon page.

The title has 55V 40A 2000W and Reverse Switch, just below that it's
2 amps, continous current 30A 25KHz and the description has 10A fuse,
max power 400W. Below the "Specifications" has rated current 8A max
current 10A, max power 400W and 21KHz.

I'd guess from reading some of the reviews it works ok for a 12 volt
trolling motor.

 

I looked at the LM317 and was surprised to find that it wasn't an LM317HV, but instead was an LM317T. Unfortunately it looks like many, if not all, of the things on amazon listed as PWM motor drivers are like this. Do you guys think I should replace the speed controller or should I add protection circuitry (something like an RC or RCD snubber) to the current one? If it needs to be replaced do you think this one looks good?
https://www.robotshop.com/en/cytron-25a-7-58v-single-brushed-dc-motor-driver.html

 

> should add protection circuitry (something like an RC or RCD snubber)
> to the current one?

I don't see how just a snubber will help. The amazon controller
appears to lack any current limit.

Do you know the current needed by your motor for scooter speeds?

Your alternate controller choice is current limited to about 25A.

Your alternative:

Cytron 25A 7-58V Single Brushed DC Motor Driver by Cytron $49.00
https://www.robotshop.com/en/cytron-25a-7-58v-single-brushed-dc-motor-driver.html

Product Highlights

Cytron 25A 7-58V Single Brushed DC Motor Driver
Features bidirectional control for one brushed DC motor
Offers overcurrent protection with active current limiting
Provides temperature protection and under voltage shutdown
Includes buttons for quick testing and LEDs for motor output state

datasheet:
https://www.robotshop.com/media/fil...-single-brushed-dc-motor-driver-datasheet.pdf

Your battery voltage is close or at this controller's maximum voltage.
58 volts / 4 batteries -> 14.5 volts/battery.

I'd guess that only under charge would the battery voltage exceed that.
You'd need to consider how you charge the batteries and if the controller
needed to be disconnected during charge.

Also the thermal limit might restrict the scooter speed based on the
temperature...

More on the motor control topics:

https://www.robotshop.com/community/blog/show/brushed-dc-motor-driver-explained-part-1

 

I'm not sure how much current it uses when running, but since the motor has a resistance of about 2 ohms, the most it could draw is just under 30 amps if the battery pack has just been taken off the charge. I disconnect the battery from the rest of the scooter when charging, so the speed controller and the charger never directly interact. Current doesn't seem to be an issue, though. Sometimes the controller feels a bit warm after the scooter has been used, but that's it.

 

I guess I'll keep using the speed controller I currently have with the addition of an RCD snubber

 

I was thinking of using a RGP30J diode. Does this look like a better diode than the 1N5402 I used before?