I've found the best result is by finding a production bike that has the same type of forks (i.e. inverted), same brake caliper bolt spacing and rotor on the same side as your bike, then buying a complete aftermarket brake lever/reservoir and caliper assembly for that. That usually works the best as you have fresh pads, the right brake line setup and pressure for the rotor, no need to bleed and a parts reorder path should it fail in the future. Failing that if you search the web for front disc brake assembly for the 50cc to 150cc dirt bikes and often you will see 3-4 pics into say an ebay one the measurements are often quoted on one picture. You might even be lucky, but remember for web brakes pad reorder can become hard and importantly take note of lever sizes and what size handelbar they fit on.. they can be big at times.
To go an OEM (or aftermarket OEM replacement), or even a setup off the web first check what sort of disk rotor you have & thickness ..most CRF disk brake setups come with forks, rotor and caliper setup in a bundle, just make sure you have a motorcycle rotor and not some thin home made mtb one.
Then measure the existing brake line length, the exact caliper mounting points distance (measure from the center of each mounting bolt) and the position of the caliper (left or right) & if your caliper has some sort of backing plate for offset (often subject to the fork tube mounting points), the caliper bot sizes, and your handlebar size. Take note of all these measurements, then start looking for a suitable donor production bike or pit bike like a Ktm Sx 50 mini or 50 senior, Pitser pro 50 etc. Get your measurements then go shopping at your nearest pit bike dealer that has a good range of 50-90cc bikes, even scooter shops have good options but levers can be bigger. Sometimes a backing plate on the rotor is a good thing as often they have plenty of space & can be redrilled to correctly retrofit the caliper to your setup.
I run a cable on my xr50 with a caliper, but run an aftermarket ktm50pro jnr complete caliper and front brake assembly on my z50r that has a skyteam rotor ... so much stopping power its crazy strong and lever is not huge, nor is the brake line.