I am going to assume you have a petrol engined 156. The timing belt is located under the black cover on the left hand side of the engine. If you have a 2.0 there will be 2 belts under this cover. 1.6 and 1.8's only have 1 belt.
The covers are easy enough to remove. Remove the aux belt and the aux belt idler (black plastic wheel), then remove a load of hex screws. The upper cover will come off and you will be able to see everything but the crank pully needs to come off to remove the lower cover.
Unfortunately if it has snapped then you will likely have irrepairably damaged the valves, potentialy the bottom end bearings too. At this stage a good 2nd hand engine may be the best way forward.
The timing belt must be changed every 36k miles or 3 years, whichever is soonest. The changing frequency was changed a few years ago so the figure in your owners manual will be incorrect.
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