I have a 33 year old New Yorker oil boiler that is on it's last legs.
I am wondering if I should replace it with an oil boiler or a propane one.
Any idea if the KERR GA100 is a good gas boiler?

Never heard of the name brand before. As far a oil or propane goes you will have to do a price analysis based upon your usage to see which one will use the less fuel. Depending upon your system and the temperature it runs at you might look into a gas/propane condensing boiler. They are more efficient. I recommend you get at least 2 estimates and pick the best company.