There is coil on receiver board. So most likely frequency is set by LC circuit and transmitter probably has same coil and capacitor, several transistors and IC. To determine frequency you need to measure inductance of coil and look what capacitance has capacitor, then calculate frequency. Or build transmitter using this coil and cap and measure it's frequency. You likely don't have frequency meter or very accurate LC meter to do so.
And this is not only problem. Main frequency is modulated by IC. Your receiver has 125c7 rx(rx means receiver) and transmitter probably had 125c7 tx. 125c7 tx modulates transmitter frequency and sends code (which is set by switches) to receiver, where 125c7 rx decodes and checks the code. So you need to find 125c7 tx or emulate it by using microcontroller(and you need how to program them), in this case you need working transmitter to reverse engineere protocol, this is not simple task by itself, or documentation where communication protocol of these IC's is described, but i couldn't find any datasheet of this IC after short googling.
It's easier and cheaper to buy new transmitter and receiver, than to try make transmitter for this