I actually got the Haynes manual for the delorean time machine. There's a reference to this, in that Doc replaces the stock Peugot-Renault-Volvo 130hp 6-cylinder engine with a Porche 928 V8 and converted the transmission into an automatic-manual hybrid.
He drained the fluids of the car before storing it. Gas goes bad eventually and would have gummed up all the fuel lines in the 75 years it sat there. (Source:https://www.amazon.com/Back-Future-DeLorean-Machine-Workshop/dp/1683836219)
Now what he did with the gas afterward? He must have used it elsewhere because after he repaired the fuel line they tried to run the car off the "strongest stuff" the local bar had and it blew the fuel injection manifold.
I love this movie. I have a BTTF T-Shirt and always get positive comments on it every time I wear it.
There is a "Delorean Time Machine: Owner's Workshop Manual" that just came out. It is hardback and is pretty cool. It is made to look like an owner's manual along with detailing how the time machine works with some parts "redacted". It also has "notes" from Doc Brown which detail little missing things from the movie. For example how Doc and Marty met. You can search for it but here is an amazon link to it. It is $27 but worth it for the hardcore fans. https://www.amazon.com/gp/product/1683836219