I suggest you avoid increasing the outside diameter of the wheels. Larger driving wheels means taller gearing, which means more wear when starting off, less ability to start on steep grades, and less convenience when manoeuvring at low speed. You need all the wheels to be the same diameter to keep the deck level when mowing.
If you put block pattern tyres on it you will improve traction but the tyres will tear up the grass when mowing. If you need to work on steep slopes or wet ground you may need to do it, but the job won't look all that great when you finish it. You might reduce the damage by using wider tyres, to spread the load.
