If they are the standard SC430 wheels, spec should read ..
PCD 5 x 114.3
Diameter 18"
Offset 45
spec should be embossed on the back of the wheel spokes 👍🏻
Have heard of another belt jumping that was a recently replaced item, all a bit odd as it has a hydraulic tensioner so sets its own tension. Let’s hope it’s not a new problem .
From memory the seat back clips in at the top and is screwed in underneath.
Take the seat fully forward, torch and a mirror ..screw each side.
….bit tricky to get at ☹️
once you have it off you can see if any parts are needed 👍🏻