What are the probabilities that my engine is seized after siting outside in Indiana weather for almost 20 years

 

Sitting outside how? In a car? Or on the ground? Were all the holes plugged up? Air filter on it, etc.? The only way to know for sure is to try turning it over. You could pull the spark plugs and use a wrench on the front to try to turn the crank. Or if its still in the car put a new battery in it and give it a go

 

If it's seized up after sitting around that long then it needs a bit or repair. Providing it's not full of water, chances are the rings are just stuck to the cylinder walls. Even if that's all that's wrong with it, the rings won't be in any kind of good condition and a rebuild should be done. It may start and run but probably won't run very well.

 

Yes , no , or maybe ... In no particular order ...

 

I'd say the odds are better that is is seized than not.

 

It's been in the car

 

I was doing a mopar engine swap once and the engine was in my garage for 3 weeks and it seized. Pull the plugs and fill with marvels mystery oil and let it sit. The longer the better. Break the motor loose with a breaker bar till it spins free. Then crank a few times with the starter. I put that stuff in a motor that sat in my grandfathers field for 20 years uncovered and it broke it loose. You don't have anything to lose at this point. Give it a try.