Hi,
Keep the belt but take a look at your freewheel, the teeth might be completely worn out which results in a smaller outer diameter of the freewheel, preventing the belt to be tightened even more.
The freewheel is not particularly wear resistant, especially if you ride when it rains and on dirty roads. See also these threads: [url]Photos of Stridas in Hong Kong] and [url]Photos of Stridas in Hong Kong]