Belt Drive?
I sympathize. I'm in the Philippines with a Bridgestone Albelt. These were never produced for the export market and arrived here as surplus. It uses a standard 14mm wide 8mm pitch synchronous belt. Continental was rolling out a version of this for the European market when they shut it down and recalled whatever was already out. While this size works well enough for Asians, if anyone heavier than the average European twelve year old gets on the pedals hard the belt may snap with a very real possibility of serious injury.
So, the first thing you want to do is determine if your frame really is, "belt drive ready", or just has a stay break to let you get a belt on/off of it. If the frame is not rigid enough it will not hold the proper belt alignment under load. If you can't get the belt it came with to stay on the cogs I suspect it isn't truly "belt drive ready" even if it was sold with one installed.
What are the dimensions of the current drive belt? (width, tooth pitch, number of teeth) What are the present cogs tooth count? Are you trying to keep the current gearing or looking to modify it as part of the change?