Is this a new engine, or used? If it’s used, the first thing is to see how well it runs on DC. Older DC engines that run poorly will run even worse on DCC.
Is there space inside the shell? For small engines like this, the issue is usually finding space for the decoder.
To install a decoder, you need to make sure the motor is isolated from the frame. Again, older engines are likely to be a problem while newer ones normally come with isolated motors.
If this is a relatively new enigine, it may well have a circuit board with a jack already installed for a decoder. Check the directions, or look online for a detailed description of the locomotive.
Minitrains are made in Germany and they have a web site. There is also a web site of the U.S. dealer. The U.S. dealer’s site provides no details about the innards of the Porter 0-4-0 and whether it’s possible to install a decoder, but the German web site has a step-by-step tutorial on how to do it.
Even if you don’t understand German, you may be able to figure out how to install a decoder just from the pictures.
Click on the German flag and then “Technik” at the top of the page. Two pictures will come on the screen, with the top one being the 0-4-0 Porter. Click on the photo and step-by-step decoder installation pictures will be shown.
If you’re using Google Chrome as your web browser, look for a prompt near the top of the page that says, “This site is in German. Would you like to translate it into English?” and a box that says, “Translate.” Then all the instructions will be in semi-understandable English, and you can even print out a copy for reference.