Some very simple things to do. Does the engine turn over with the electric starter? The starter will need to work before you can check for spark. If it turns over and you don't hear any horrible noises, then the cam chain is probably timed right. Remove both plugs, put the wires back on them, and lay them on the engine so they are grounded. turn the engine over with the starter. It should turn pretty fast with the plugs out. You should get a big fat blue spark at each plug. It helps to do this check in the shade, out of direct sunlight. If you get a good spark from both plugs, the ignition system is fine. The spark at the plugs is the final step in the ignition process. If you have a good spark, everything from the plugs back is ok. If the spark is good, do a compression test. You should get close to 160 psig compression. It might run with less, but not very well. If the compression is low, you have a mechanical problem inside the engine. If you have compression and a spark, and the valves and ignition are timed right, all you need is a proper air fuel mixture, which is the job of the carburetor.
As was said, ignition timing is not adjustable on the Rebel. I don't know about the Rebel, but I have been able to test the pickup coils (that trigger the CDI and provide the spark) on some bikes using a couple of 3V DC LEDs. If you connect the LEDs to the wires that go from the pickup coils (known as a Hall Effect sensor) to the CDI, and turn the engine over, the LEDs should flash.
There are a lot of things that can prevent current from getting to the primary side of the ignition system. The ignition switch, kill switch, clutch nanny switch, sidestand nanny switch, or any wire connection could be bad, breaking the ignition circuit.
What puzzles me is that the seller said the timing was off. I wonder what he meant? You're just going to have to get a manual, a multimeter and a compression gauge, and track it down. A low or no compression reading could mean the valve timing is off, but not far enough for the pistons and valves to collide.
Sounds like an interesting problem. I'd like to get my hands on that bike myself. I could at least figure out what the problem was. I like solving mechanical puzzles. You need to do methodical troubleshooting, and find the problem before throwing expensive parts at it. Take it one issue at the time. Either confirm or rule out problems as you go.