The Traxxas Stampede 2x4 most likely came with a Traxxas Titan 12t 550 brushed motor. That motor can be found on numerous Traxxas vehicles with brushed motors including the 2wd Slash and the 4wd 1/10 scale 4-Tec 2.0 on-road vehicle.

The number you see most often on brushless motors is the KV rating of the motor or how much power the motor has. Please refer to page #10 of this .pdf from Traxxas that list the specs for the Velineon 3500 motor.

https://traxxas.com/sites/default/fi...X_manual_0.pdf

Here are some basic specs if you don't have the ability to view the pdf file...
Velineon 3500 motor
- Sensorless
- RPM/Volt: 3500 (10-turn)
- Max RPM: 50,000
- Diameter: 36mm (540 size)
- Length: 55mm

Hope this info helps.