Oh, this is too cool! Make your lawnmower as fearsome as a NASCAR speedster with a racing car-style paint job. We're talkin' flames!

Find out how by visiting this Instructables post by Sam. There's a truly excellent slideshow at the site, demonstrating each step of the project. Basically, it involves the following:
1. Remove the lawnmower's engine cover
2. Sand and prime it
3. Apply a base coat. Ideally, in a color that creates a suitable contrast with the red, orange and yellow flames. In this case, the chosen color is bright blue
4. Apply a layer of painter's tape all over the area where the flames are to go
5. Draw out the flames with a pencil, then score their outline with a blade and peel those pieces off
6. Spray on the flames in red, orange and yellow. I think these guys airbrushed the flames to create a smooth, naturalistic look.
As you can see from the photo here, it looks totally professional.

Common sense tip: with all that spray painting and closeup work (drawing and scoring), good ventilation is a must for this project.

Sam: what can I say? This idea bears the mark of genius.