i would definitely get a print out of the auction, and any and ALL information on the seller (place of business residence, name of Owner, any one you talked to, dates and times of meetings/phone conversations etc).
If i were you i would gather all this information.
Also, if mazda denies your claim, (as im sure they will) if i were you i would get an attorney in the area, or even one close to where the Auctioneer's (car seller's) main operations are and get him to write a letter to the seller of the car demanding payment. THey will deny it and then you might be off to court.
One issue that the seller could bring up, is that he could "discover" all the info mazda received when they read your ecu black box's last few seconds before it blew.
If it says overboosting, then the seller could assert that he warrantied the car in "normal" use however he did not represent to you that the car would be warrantied under "overboosting" or "abusive" conditions.
Either way, You are going to have to pay for an attorney, or just shell out the cash, get a used motor, and then go from there.
I had the same fiasco with my msp, and needless to say my dad is a lawyer and Mazda strong armed us, and i just sucked it up and bought a new forged engine.
I think the evidence that your engine was overboosting when it let go will hurt you, though it will not necessarily make your case fail.
I would be interested to see what happens.
CHEAPEST WAY would be to get a used motor and have a aftermarket mechanic install etc...you will lose your warranty, however you really never had one in the first place.
OR, you might ask the seller of the car for the amt. to buy an engine (used) and threaten a lawsuit for breach of express warranty if they dont agree to that...