https://www.micra.org.uk/index.php?threads/61943/
You'll have to look through to find the pics and info.
if you can find a number on the belt then just go ask for one of those.
If not just take it off and take it into the store...
failing that the belt numbers are generally of the form QPKRXYZ where Q is the number of ribs the belt has and RXYZ is the length of the belt in millemetres. eg 4PK0990 would be a 4 ribbed belt 990mm long or it may be length divided by ten I can't remember...
Pray to god, but row to shore...
I'm not really sure what you're having issues with, you took the belts off right? put them back on the same way.
I generally judge a belt to be tight enough if I get less than say 6 or 7mm deflection over a reasonably long run (gap between pulleys) there's probably something in Haynes...