It looks to me like you have a lot of room in the neck pocket for some adjustments. You might be able to get away with either scale length. If the guitar was originally 25 & you try a 24.75, that might not work because the neck with have to fit farther back in the mortise and may take some of the space for the neck pup.
Basically what I'm getting at is that you could slide your neck back & forth in the mortise to set the scale length.
There's a guy on ebay selling kits, but I don't remember the scale length. But the pics BlackLantern posted look exactly like 'em.
Exactly.
As long as the nut is the correct distance from the bridge, he'll be fine.
For all intents and purposes the body is a clean slate. *IF* the OP can make a wedge that brings the neck to the proper angle, and supports the heel enough, and doesn't look like ass, he'll be fine.
Is it the ideal way to build a guitar? No, and I don't recommend it, but skillfully done, it will work.
Umm, actually it would work, but the window for being right is about .01" (give or take the leeway afforded by the saddles in the bridge). A bridge is just a fixed point. This is why you can buy baritone scale necks from Warmoth, and put 'em on your Strat. The question is whether or not you'd have enough support under the heel, and if you'd lose any space for pickups.
yeah, it definitely looks like it was built for a 24 fret 25.5" scale neck

ah the plot thickens....
you should just ask your customer where he got it from.