IMHO NO!

back in the day there were plenty of gas to Kerosene conversion kits available.

More than likely you did your brother a favor. Remember all the folks that poured ATF in their carbs to "lube the top end" of the car engine? Kerosene would be in that same category. If it ran it's all good.

What was the ratio of kero to gasoline? I wouldn't think a large percentage of kero would even run the engine.

If you didn't notice it smoking like a freight train,the ratio was not real high.Also,if the ratio was really high on kero,you would have had trouble shutting it down.I seriously doubt you harmed anything.

Baymee,

With the old gas/kero conversions the critical factor was the temperature of the kero and engine. You would start up with gasoline and warm up the engine, then switch to kerosene. I think the conversion kits had a length of metal fuel line that ran along the exhaust manifold to pre-heat the kero.

I am guessing had this happened in the winter it may not have worked!

I drained his tank ,put fresh fuel in and replaced 14 year old spark plugs. The mower has 200 hours on it , he push mowes his half acre and it is amazing how it looks. The deere is for when he is short on time. The engine is super clean from looking in the plug holes with a light . I put 1 gallon of kero in to a 3.5 tank, he was using this mower for a long time with the choke stuck slightly repaired that. The single choke and throttle is bad , im glad mine has seperate ones. Oh yeah one more thing, he told me , in my hand mower I use two cycle oil and gas to get rid of it, my mower smokes ..just a little LOL. I got in my car and left after that comment good day .....
thank you gentlemen as usual ...

I'm told that back in the early 1900's, gasoline was considered a waste by-product of the crude oil refining process that produced kerosene. Kerosene was in demand because it was a good replacement for whale oil, which had been used not only as lamp fuel but as a high grade lubricant for delicate machinery. Whale oil was becoming increasingly expensive and hard to get because... of course it requires harvesting a whale to get just a few liters. Kerosene became the cheap and plentiful alternative fuel.

The Ford Model T could run fine on straight kerosene, if it was first started and warmed up on gasoline.

In the aftermath of WWII in Europe, their refineries had been largely destroyed by military action, and very little gasoline was available. In post-war recovery, England had gasoline tractors converted to run on something called "TVO" Tractor Vaporizing Oil, which was sort of a volatized kerosene (3 parts kero to 1 part gasoline, if memory serves). It allowed English farmers to get their tractors running and get food production back online after the war.

So yeah, as others have described here, there would be no damage to the engine from that kerosene you put in.