The old tensioner is unlikely to rattle - more likely it will start to squeal when worn out.
To answer your question though, what sort of mileage is on the clock?
From personal experience those tensioners last forever but you start taking a risk after 100k or 20 years. Given the car is definitely over 20 years old I would say that qualifies just on age...
The risk is a belt snapping and you having to reset the timing - these are non-interference engines so if the belt does go nothing gets broken (in theory).
In terms of replacing it - jack up the rhs of the car, take of the wheel and remove the cambelt cover (this should be a guide in its own right) and rotate the crank pulley by hand (with the help of a spanner/wrench) and get the crank *and* cam pulleys aligned with known timing marks. If you can't see an obvious one, don't be shy of making your own with a bit of white nail varnish - just don't mark the crank pulley on the outer face as it will make strobe timing the engine rather tricky unless you use the official timing mark on the pulley.
With the wheels chocked, handbrake on, in gear so that the engine won't turn by accident you can start.
Loosen the timing belt tensioner and slide the old belt off, put it in the bin.
Clean the pulleys off carefully (just use a dry cloth - don't use solvent) and make sure there aren't any oil leaks near where the belt runs. Assuming all is clear you can start putting the new belt on, or swap the tensioner first if desired.
Note that the tensioner is partially sprung thanks to a plunger on the engine mount which helps to take up any additional slack that the timing belt develops (something the later fiats could sorely do with) and assists in avoiding the belt jumping a tooth.
The new belt goes on (my personal method) from the crank pulley, working against the direction of rotation, over the cam pulley - making sure the timing marks are still aligned and the belt is roughly tensioned between the two. The return loop of the belt can be a little slack as the tensioner will take that up. If you do it the other way around you can pull the cam pulley around by accident so unless you have a locking tool to stop it turning it can get pretty frustrating.
Once you've got the tensioner tightened up you can check the timing belt by twisting the tight side by hand - it should rotate at the mid-point by 180 degrees so the teeth are facing the wrong way but no more than that. If you can't turn it that far (by hand) then the belt is too tight.
Rotate the crank pulley by a full 720 degrees (back to your timing mark) and re-tension it. For obvious reasons the crank and cam pulleys should be aligned with their marks - if you only rotate the crank once the can will be off by 180 degrees...
If you are going to strobe time the engine as a check it is worth putting a dab of tipex of white nail varnish on the official crank pulley timing mark and on the timing marks on the cam cover. The best place to time the engine is actually at the other end using the marks on the flywheel and the bellhousing but it can be a real bind to find them in the first place if you don't know what you're looking for (another guide in its own right again).
Skipping over the timing bit you should now be ready to reassemble the cover and put the wheel back on, put the car on the ground, put it in neutral and try starting the engine.
Don't rev it up straight away, give the belt a chance to settle, stop the engine and check the timing marks again. Once you're happy with the results you can take the car for a test drive - just keep in mind that there is a slim chance of failure early on (very slim) so don't go mad.
On this subject I suggest getting a proper "gripper" branded belt as these come with a warranty that covers potential engine damage as a result of the belt failing. It isn't so important on these engines as there isn't much to get damaged but it is an extra safety net. I don't know of any other brands that come with a warranty like this!
Once you've got a few miles on the clock you can start to drive normally - if the belt is going to fail it will be in those first few miles or at the other end of its lifespan.
