No one should be paying $50 per plug, that's lunacy.
What would be the correct plug for this engine?
Champion CJ8 or (better, and my preference) NGK BM6A
Michael
I don't know much about Tecumsehs but I am guessing 3.5 HP means it is a flathead. If you look at your old plug and note its thread reach, you can basically tell by looking which ones will fit. They have a shallow reach into the combustion chamber compared to an OHV spark plug. *According to the link below the reach should be 9.5mm.
Must be hundreds of suitable plugs. This cross reference site may shed some light on the subject (the link shows a search of plugs comparable to Champion RJ19LM):
Sparkplug Cross-reference
Of course you have to do independent research on each one to find out the heat range and materials they're made of (copper, platinum, iridium).