It would be helpfull to have the service manual. I've done it in cassette tape decks without any service manual help. But Reel to reel is not know for me.
You will have to figure it out how to get to the belt. It shouldn't be so hard, even without service manual. Be very, sctrictly, disciplinated when putting screws and parts aside not to get messed up. I use to take some fotos during the dissasemble to recover from "I don't know how to re-assemble this thing".
I've always used aftermarket belts but a more expert opinion would have to be heard to sentence on this.