I've used Milton sterilising solution before on my mx5 soft top. I've also used diluted white vinegar which worked well, but took a while for the smell to fade.
You have to get rid of every spore or it will come back eventually. As it's a project, are you able to remove the seatbelts altogether to properly clean and dry them? You don't really want to be getting them wet and then roll them back without being thoroughly dry.
I have an ongoing yearly battle with my wife's MGF with mould. It's impossible to get to all of it with the roof on, so every year as soon as it gets damp outside it slowly creeps back