The don't make very good ESC's because they only pulse at a maximum duty cycle of 50%, meaning your motors only receive half the current they should, limiting their power.
I wish I had known that before I used them getting the full function all those years ago. The only thing you miss out on is the lack of deadband. A servo has the exact same logic elements as an ESC, but a couple of components have their values changed to make them more suited to keeping a position rather than providing running control. No problem using them singly, except that you are limited to the radio battery and half an amp for the motor plus the lack of a positive stop.
Using a couple, they might not respond well to the mixer, "proper" ESCs, being what the mixer was designed for, will be much better.
The modification consists of removing the gearbox from the top of the servo, removing the bit of moulding covering the motor shaft, maybe getting the gear off it, plugging in and setting the "stop" using the exposed potentiometer shaft as a trim. The result will be fully proportional, BUT it will be best to control the speed of the motor using the trim tabs alongside the sticks as an engine room telegraph. Full speed appears very early in the stick travel.
Optionally, the motor can be replaced, but it needs to be a close match to the one in the servo. The original can be dug out to mount elsewhere.
If a bigger motor is wanted - either more amps or more volts or both, it is easy to make a higher power output circuit for the servo electronics to drive. Nowadays, it is actually cheaper to buy a ready made ESC - the result will likely be smaller and lighter.