Oil level should be between add and full marks.

Remove the spark plug before trying to turn it and see if that helps.

Posting the engine Model & type would be helpful.
Saying "Has the industrial/commercial engine" isn't very useful, except that it's a Briggs and you mentioned 12HP.
WHICH Briggs 12HP IC?

Do you know the approx year it was built. Engine code will show when that was built.

Where exactly can I find the engine code?The metal tag on the rear of the body says manufacturing date 2971.On the engine it says 12 hp i/c snycro/balanced.

Briggs Engine numbers should be stamped/etched on an exterior sheet metal part on the engine.

You can also check out the Briggs Model number identification video at the link below...

Here is a link that might be useful: Briggs manuals

Well mich, I should have gone further down the list before I responded to your post in the other thread. As a preliminary suggestion on your problem, you need to at least remove the spark plug from the engine and empty any oil or water out (if you can) and then fill the cylinder (through the spark plug hole) with some kerosene, diesel fuel, mineral spirits, auto trans fluid, Marvel Mystery Oil or any other type of thin oil or solvent (except don't use any highly flammable stuff like gasoline or lacquer thinner). DO NOT put the plug back in. Now, let it sit for a day or 2 and then see if the engine will turn, leave spark plug out until you get the engine freed up (if you can get it to turn).

Thank you for getting back to me,I'll try that and let you know if it works.I also found the model# above the spark plug using a mirror.It's 283707, have also downloaded the briggs and stratton manual for that particular engine.Here's hoping,the mower is in remarkable shape to for it's age.Thanks again and I'll get back to you on the results.