Help: Marin Top Stem Lug(?) Sheared

I have an *old* Marin Palisades (1993?), and am having difficulty with
the handlebars...

The top alloy insert into which the bolt holding the handlebars tight
sits (threaded headset) - has sheared in two (due to overtightening) -
leaving the headset bolt untensioned (nothing to support it in tube!)

Short of finding a large washer(!) - What would the best remedy be,
please? - as I don't want to replace the stem (it's nicely colour
co-ordinated - luminous yellow - with the front forks and
handlebars...)

TIA,
Aitch