Doesn't sound like a seize since you were able to re-start it. Sounds more like bottom end. Probably due for bearings/seals anyway. I have a set that I have been planning to put in my M56 this winter, just haven't got to it yet. Here's the list of bearings/seals you'll need: https://vintagemopeds.net/batavus-laura-m56-engine-bearing-seals-rings/
If you have trouble finding them, check with a local bearing shop. They can usually source them, if not let me know and I can probably find you some.
They also have a lot of manuals and stuff there for M56: https://vintagemopeds.net/manuals/
Make sure it has gear oil when you rebuild it (be interesting to see if it had any in it right now). Also, as suggested, you should probably check your jetting on that SHA. I doubt the stock jet is correct.