I have an engine which is acting up. It appears to be a bad spark plug. The plug that was in the engine is a J18LM Champion; these appear to be in short supply at the present time.

Is there an equivalent plug (number?) that could be substituted?

If you try to upgrade you rplug (i.e. spend more money) usually they will not have any issue finding you an equivalent plug...

The j17lm will work or add an R after it and get a resistor plug. The other would be J8J
Regards,
Don