Fibre glass, maybe with some mesh reinforcement, should be ok around the skirt but if there is rot on the top where the blade bearings are mounted I suggest that metal and welding are the order of the day.
Your Mountfield is basically an American Murray so that spreads the net a bit when sourcing parts.
As has been suggested, Garfitts are a possible source of chassis parts – they list a blade G22-031, original Murray part 20083 but you need to be sure about the centre hole. From memory, some were round and were rectangular with rounded ends – known as “double D”.
I’m guessing that it has a Briggs and Stratton engine ? No problem with BS parts. belts should not be an issue but some machines only functioned correctly with Original Equipment belts and were difficult with cheaper ones of nominally the same size from local belt and bearing merchants.
Talking of bearings, I seem to remember that the cutting spindle bearings were OEM specials and difficult or impossible to match with cheaper off the shelf items.