Don't underestimate the fire risk to the tractor, mower & grass you are mowing all are potential ignition points - Cutting grass & particularly long paddock type grass throws/accummulates dry debris/stubble on the tractor & the mower, combine that with a hot engine/exhaust/PTO/gearboxes (including the mower) + the inherrent risk in dry grass of mower blades hitting rocks & sparking to ignite a grass fire, or a grass fire due to the static discharge from the machinery ....etc.
Experience has taught us as good practice on our farms (& as a condition of our insurance) for all self propelled/powered machinery (tractors, trucks, harvest/hay making/earthmoving gear, ride on mowers ...etc) to carry fire extinguishers - a small investment which with quick action can prevent a catustrophic result.
Similarly depending on the extent/duration of your mowing activities it is good practice to clear the machinery sometimes several times a day (& always at the end of each day ) of all debris (we use compressed air though a lead blower can suffice) & inspect/clean the air filter minimum daily.
Some further thoughts, & whilst not wishing to scare you off contracting as there's good money to made:
- We contract as well as operating our own farms & personally there is no way I'd commence any contracting (however small) considering today's legal/liabilty obligations without comprehensive public liability insurance ( you'll generally be liable for all damage when contracting - accidently start a fire with your tractor/mower & you could be up for a $M bill for all consequential damage, same if you injury any people/property as a result of your contracting acctivities...etc).