I know the feeling. These ebikes are great, but it can be pretty frustrating in the beginning. Your next step will need a multimeter. a very cheap votlmeter will help with sorting out polarity of things and seeing if there is current at all.
Here is how to test the temperature sensor in the motor. The sensor is a switch that is on when it is cool, and off when it gets hot. If it is working you will see voltage going through it. So you put a battery on one wire, then the voltmeter on the other wire, and the other end of the voltmeter back to the battery. If you see voltage, the temp switch is ok.
Unfortunately I never figured out how to put a link into this forum. But basicly ebay has the cheapest options. If you search for scooter controller,, there is a good looking one from a vendor called morethanhundreds. I bought a charger from him, or her, and got it very quick. What you are looking for is a brushed controller, 24v volt , 500 watt. Over 500 watts may make the motor run hotter, and under 500 might be slow. But ANY brushed hub controller 24v will work
Searching for scooter throttle, a vendor called tnc scooters has a 24v throttle with led lights. The led's will help you know it's powered up, and when to stop riding.
What I am using is a Wilderness Energy bike kit, brushed controller and throtte. It's a bit overpowered for the motor, so I have to watch out or I'll overheat the motor. It's actually a 36v controller, but it works at 24v too.
Other places to get brushed controllers are scooter part vendors like monsterscooterparts and electricscooterparts.
Hopefully you will find a loose connection somewhere and fix it. I am not expert enough to know how to diagnose a throttle, and all I know about controllers is if they stink, the magic smoke must have leaked out.

But at least your problems should be under $100 to fix. The motor runs and that is the really important thing. At 24v it is possible to just put a heavy duty on off switch on the handlbars and let er rip. At higher voltage, the switch just melts if you try that. The switch works, but it is hard on batteries and controllers, so don't resort to that for more that a short time, and be carefull not to melt down the motor. If it feels pretty hot where the wires come out, stop. I melted a motor in one ride, by running it full throttle nonstop too long.