I bought some Celestion v12 speakers that are in a Crate GXT100 yesterday, and the guy just gave me the whole amp and said it didn't work. I plugged it in, but the transformer started smoking after 30 seconds, and there was a thick bundle of wire in place of the fuse. Apparantly these amps are notorious for bad transformers and transistors so im not going to bother trying to fix it.

I want to make the most out of this, So im hoping to make a pedal or two out of it?
It has a reverb unit and two stock 12ax7's in it.
I want to kill my brother with kindness and make him a reverb pedal for his' 67 Bassman, since he's been such a dick lately.

Is it possible to make a reverb pedal with 1 volume and reverb level knob?
How difficult would it be?

Also, would it be okay to replace the wood panel that holds the speakers with particle board? I want to make it look like a decent 2x12 cab rather than a stripped combo
this kind

You can use particle board for speaker cabs. It isn't as tough as ply but has better sonic qualities. You need to know that it comes in different grades. The top sample in your pic looks like three layer board where the outer layers are made of finer particles and contain more resin. This is what you need. It also comes in different densities dependant upon the pressure used to roll it out. The veneered second sample you show is low grade low density stuff, you can see air gaps. This is too soft for your use. Even the top sample looks like medium density to me though the photo isn't clear. Over here high density stuff is readily availble and cheaper than ply, I wouldn't hesitate to use it if I were covering it. Don't use MDF though, it is useless if it gets damp and is too heavy.

You can use the spring unit for the reverb but I wouldn't bother re using the other components unless you know how to check them. You can check the transformer by entirely disconnecting it from the amp and connecting it to the mains (only if you know how to do this safely) smoke means it is shot. If it runs without blowing fuses you can check the output voltages. If it is OK then the problem is in the amp and you could check the power supply components and the output transistors.

Don't do any of this unless you are really sure about safety. The transformer gives voltages that could kill you and the power supply will store those voltages even after the amp is switched off and unplugged.
I'ver got a dead amp i'm trying to fix with a reverb tank too. I might do the same... Keep in mind the reverb unit doesn't work by itself. You need some circuitry to go with it which should be located on the main board, making it hard to use. Here's a link to a spring reverb circuit: http://www.paia.com/ProdArticles/hotspuse.htm You could possibly modify it to make it run with tubes too.
The pedal would be pretty big, but it's definitely possible, you would just create a buffered effect loop pedal, run the reverb in place of the FX loop. You could use tubes instead of FETs but it would make the pedal larger and require more circuitry for the heaters and such.

It might be easier just to turn it into a tube buffered reverb tank.