Ok, here's the thing, i have an old carlsbro acoustic amp, with a Celestion G12 1545 Speaker.
It worked kinda well for my electric guitar, but recently the amp just died, but i guess the speaker is good, so i'm wondering if i could use it as a speaker for a amp head - like building a cab with the celestion in it. ( i'm thinking of buying an orange thunder 30).

Sorry I'm still a noob so I have almost no idea what I am getting into now lol.
Are you sure it is the amp that is broken and not the speaker? Does the speaker work with any other amp? Have you tried the amp with other speakers. If by dying you mean nothing lights up, have you checked that there isn't a fuse inside the amps casing?

If you still want to go ahead you just need to make a box the same size (volume) as the box the speaker was originally in.