Hi! My question would be if i can connect an 1/8 brushless motor to a 1/10 70 amps ESC (the onyx one). Problem is that the 1/10 motor gave up and im looking for something that is more torquey/doesnt heat up as easily. Or, do you know any 1/10 motors that are, say, more torquey than the rest?

You should include what kind of truck you have. Way easier to recommend electronics. Putting an 8 scale motor in a 1/10 scale truck with a 70a esc would be ill advised. You can find a whole slew of esc/ motor combos specifically designed for your truck. The thing is, when you start going faster, heat is a problem as you know. Not just on the electronics but everything else. The diff being an important part you dont want to heat up or break. Most esc's have a max cell/ turn/ kv rating. Very important you follow those recommendations. Stressed electronics with high voltage batteries is not a good mix.
I would go online and do some research on esc/motor combos. There are real fast and waterproof combos out thede that will be a good fit for your needs..One could go on and on and on and on...about what combo is best. Im not getting into all that.
Just remember, you get what you pay for. Look at lower priced brands like Leopard and Gool rc. Hobbywing also makes some good stuff. I dont know what you have but maybe look at some Traxxas electronics. The VXL series electronics is what they use for there fast brushless vehicles.