Is there an actual spark at the plug gap when its pulled out of the engine and checked while cranking?
I don't know what make/model engine you have, but many small engines, even ones 10-12 years old have electronic capacitor discharge ignition modules that can be destroyed by polarity reversal. In that case there will be no spark at all, and the module has to be replaced. The voltage regulator/rectifier and/or alternator stator can also be damaged, which you might discover after getting the engine running that it will not charge the battery at all. The fuel shutoff solenoid is also a DC component, but I tried polarity reversal on an old one I had in the shop and it was not damaged. Not trying to scare you, just throwing out some ideas.
The easiest way to distinguish between fuel or ignition no-start problems is to spray a little gumout or starting fluid down the air intake and try to start it. If the ignition is OK, the engine will usually start and run for a few seconds and stop, this indicating a fuel delivery problem. If it does not start at all, it's often ignition system at fault.
-Fordlords-