^^ what cliffy said
you will need to work out what engine it is and find the service manuals.
Briggs and Stratton have them on line. Unfortunately Honda do not (you have to buy them from America)
There is a model number someone on the genset showing what enginer it is, worth googling for this.
Then find someone with a decent socket set, and a garage with enough space and lack of interruptions (i.e without loads of people crowding round, fag/spliff in hand) and a few litres of paraffin (to soak the various oiled bits in to clean them) (if you have any gary boy mates they may be able to help you here, assuming that todays gary’s actually do know something about engines and not just bolt-on accessories).
I suspect the fuel filter has got contaminated with the sump oil and that will need replacing for a start. Maybe even other bits and pieces (mostly filters)
There are lots of gardening/outdoor type places round our way though (particularly in Woodbridge/Nacton) who may be able to get the parts. (or you might strike it lucky and find some old boy what worked at Ransomes or similar place who would fix it for you for a few quid…)
