I had this happen on mine, and I fixed it Home Depot style.
I used an L-shaped mending bracket from the hardware store, and drilled holes in the remaining almost-vertical stump and riveted the L-bracket onto it. The L shape gives me a horizontal piece that only needed a little grinding to re-fit the plastic handle on.
Here's what I did, posted this on mazdatrucking.com, only problem is that the photos were lost in the site crash. It looks pretty darn good, may have to take new photos to add here.
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OK, fixed it. I used a 2-inch flat corner brace from Home Depot.
Here's the part that broke off; I did bend it a tad over the years as my kids kicked it in, so that likely accelerated its breakage.
Here I'm holding the corner brace over the remaining seat lever part.
I ground down the one "arm" a little to fit into the gray plastic handle, and tapped it in.
I drilled two 1/8 inch holes in the remaining part of the lever, to align with the two holes in the corner brace (I slid a piece of wood in behind the broken lever to protect the seat from the drill bit). Then I sanded the corner brace a little and spray-painted it black.
Holding up against the broken part
I slid the gray cover on before installing the rivets because I already had the gray plastic handle tapped on. Photo with the two 1/8 inch steel rivets in place but not squeezed tight yet.
Photo after installing the rivets
I painted the rivet heads, and here's the final result.