Can I replace the thermostat with an aftermarket thermostat?

You can use any part that you want to that fits this engine but the OEM parts are generally better quality. This is not always the case but these parts were made specific for the vehicle and the aftermarket ones are basically just a copy of the original ones.

With a thermostat I would not worry about it and go with the one that has the price you like. However, before you install any thermostat, you want to test it by putting it in a pot of water and boil it on the stove to make sure it opens.

You can put a thermometer in the water as well just to make sure it opens at the specified temp. Normally they are around 180 degrees.

https://www.2carpros.com/articles/how-an-engine-thermostat-works

Below is the process on how to replace it if you need that as well.