A picture of your machine, and a picture of your electronics will help us advise you on making the change. Which version of the MPCNC did you build…Burly or before? Are you using a Rambo board or something else?
Step 1. You will need to decide whether you are willing to upgrade the firmware. Given the age of your machine, you will lose any custom settings when you upgrade. In addition, the firmware maintained by V1 is set up for lead screws, and a machine of your specified age probably uses threaded rod for the Z axis. It is highly likely that you can get upgraded firmware running on your machine (perhaps with a bit of forum help), but I would expect some work to getting it to run correctly.
If you elected to not upgrade the firmware, you will probably have to use M106/M107 to set the PWM on a fan pin, and then use that pin as the PWM input to the laser module. This may require some additional electronics since, for most boards, the PWM on fan pins is ground-side switched. When using fan pins, both cut quality and engraving quality will take a substantial hit when compared to inline laser commands. Inline laser commands require the firmware to be upgraded to something recent, preferably the latest version that V1 maintains since that version contains substantial laser work compared to the previous version.
Step 2: Figure out a mounting solution. Note that laser work can benefit from an active Z axis, so instead of replacing the Z axis, you may want to consider making a mount for your current Z axis. When making multiple passes cutting stock, it is often beneficial to lower the laser a bit on each pass. In my opinion, laser modules are far simpler to mount than routers, but, since your machine is so old, I don’t know if you will find a read-made mount on Thingiverse. It may be that you can use a laser mount for the Burly or even the Primo. I know Ryan kept the mounting hole distances on the Z tubing the same between the Burly and the Primo, but I don’t know about older machines.
Step 3: Hookup the laser. Looking at the pictures of your desired module, hookup should be plug and play. My laser on my Primo uses a similar companion board, and, other than having to lengthen some wires, I just had to plug everything in. You will need to figure out how you want to run the wires and the air assist hose…and you do want air assist even if it is just a cheap aquarium pump.
Edit: If you have a Rambo board, V1/Ryan’s latest firmware has laser support enabled, so you can upgrade using the binary file without needing to compile the firmware.