I'm surprised it took Thiel to bring it up - but yes, most Elco boats were reading for scrapping in 1945. They were not designed to last long, and there's a reason why their spiritual successors were for the most part built from aluminum (and even then you have cracking issues).
Why do an Elco boat? There are so many much more modern, much larger, equally cheap aluminum hulls out there from both East and West. I can link you to a few on the database that I think are appropriate.
And for those hulls you have options - though unless it were pretty decent-sized something like Harpoon is going to be ridiculous. Penguins and either a remote weapons station or L70 Bofors (the postwar version).
And if i remember correctly those engines where relics from WWI.
No! Most of the "Liberty" engines, IIRC, were re-purposed or scrapped by, say, 1924. Most of the PT Boats had brand-new Packard Merlin engines, and usually two if not three of them. These are 1,200 cubic inch displacement engines and from most of the sources I read, they had plenty of power (at least for raw speed).