I have said this a thousand times but let me say this again.
There is just no way to fully warranty a performance race engine.
Now don't misunderstand me, if I had the money and desire I could design and build a motor that would sell for under 300 dollars, last at least 10 gallons, be cheap to rebuild when it did wear out and offer a 1 year FULL REPLACEMENT warranty.
Sounds great right? I should build this thing and all of you would buy one right?
No, none of you would buy one of these motors.
Why? Because it would be slow as a turd. Oh it would run perfectly but you wouldn't be winning any races with it. As soon as you start pushing the performance envelope you open up the possibility of failures. Failure rates that are to high to be able to warranty your product.
To the OP. Your used car dealership does not apply. You are not selling a performance part. Would you warranty a race car? Also, you mention helping out your customers. That's great but what do you do if the part you need to fix the car is not available? Are you going to give the customer another car? And remember, it will have to be an equally attractive or better car to keep the customer happy.