Motor mount. Let's start with the front motor. The blue motor mount itself is not threaded for the front motor, the screws go through the blue mount and thread into the motor itself. One of the screw holes in the blue mount is actually a slot that is in the shape of an arc and is used to adjust the pinion and spur gear mesh. The rear motor mount is basically the same thing except there's the aluminum piece that's between the motor and blue mounting plate. That aluminum piece is what moves to adjust the mesh and the screw that holds the mesh threads into the aluminum piece not the blue motor mount.

Now as for a mesh screw, there's no screw that you turn to adjust mesh. You loosen the screws, adjust the mesh and tighten the screws back down to hold it there. None of these screws thread into the blue motor mount, they thread into the front motor itself or the aluminum piece for the rear motor. So the blue mount has no threads in it whatsoever.

Now if the mesh between the pinion and spur is too tight this will make the motor work harder. More work for the motor equals more heat. You can use the paper in between the gears trick or whatever you like to set the mesh, but there should be a tiny bit of play between the pinion and spur. If you hold a finger on the pinion to keep it from moving you should be able to rock the spur back and forth a little bit.

As for the temps vs speed. I hear what you're saying that you wouldn't want it to be slower. That's the problem with brushed motors in this size vehicle, they're on the limit to make it fast enough to be fun. The best thing is to just go brushless and recoup a little $ selling the stock esc and motors.

As for a kit to use your 10.5t motor. You really should stick with a 1/10 2wd buggy, stadium truck or a 1/10 4wd buggy. These types of vehicles are the perfect weight for your motor. Larger heavier vehicles like short course trucks (even 2wd) are a bit too heavy for a 540 size motor and I wouldn't feel comfortable recommending them. There's tons of 1/10 buggies, stadium trucks and 4wd buggies out there. Team associated, Losi, Durango, Kyosho, they all have at least one of each type of kit with much better parts availability than HK. Then there's tons of Tamiya's, everything from the rally cars, buggies, trucks, etc. they're more scale and a little less capable performance wise but it all depends on what you're looking for. Check and see what your local shop carries the most of if that's where you're going to be doing your parts shopping.