Only the later 1.2 70hp are unsafe, or any of the 16v. So the Sei 1.1 is safe.
Don't just fit a new belt. Find out why the old one came off. Age might well be the reason, but don't assume.
Check the tensioner, it may be seizing, and the water pump, bearings may be getting tight, or loose. Best bet is to replace tensioner and pump anyway. Timing marks are simple. There's no Haynes for the Sei, but the Cinq one will do for most things.