2manyrocks,
Body filler alone will not work for a permanent repair.
If youæ±*e budget conscious or just enjoy working and restoring parts for your Yanmar, I have had great success doing the following:
Remove and wash the entire wheel with Acetone (or denatured alcohol), clean the affected area with a wire brush thoroughly, wash again and mix and apply JB Weld or equivalent two part epoxy and let cure. Once cured, use 80 grit abrasive paper to cut, feather and shape the epoxy. Use a thin coat of polyester putty (body filler) skim coating that area (and any other scrapes and deep scratches) and finish sand with 180 and final with 320 grit paper. Prime and paint with a high quality aerosol paint (even rattle cans will work well).
My steering wheels have so far, lasted better than four years regardless of the elements and extreme Texas exposure from the sun.
Mark