What's the heater doing/not doing, exactly?
I find that ignitors are rarely the problem.
It's usually either:
1. The flame sensor (It'll light, run a few seconds, and then shut off), or
2. The rotor broke. They're graphite and tend to break fairly easily. The fuel is pump via vacuum from the rotor. If the motor kicks on, then the ignitor, but then shuts off a few seconds later, it's usually the rotor. There are 2 plugs on the back side of the machine (usually behind a plastic cover). If you find that you suspect it's option #2, pull the plug that sticks out the furthest. Put your finger over the hole and turn on the heater. You can usually feel positive pressure against your finger if the rotor is ok. If you've got a gauge, it should be anywhere from 4.0-4.8psi depending on the size of the heater. The other 'plug' sets your pump pressure.
Rarely have I encountered much more than either of these two. I've had to replace a board once, because the customer liked to pour kerosene all over it. Those are about $100. I usually replace the input and output filters on every machine I service just because no one ever does it, and they're cheap.
There really isn't a whole lot too them once you get inside.
1. Check motor. If it runs, go to #2.
2. Check for ignition. You can see it or hear it clicking. If that's good, go to #3.
3. Pump pressure. As described, check to make sure you've got pressure coming from the rotor. If not, it's broke. On the off chance, the vacuum line might be cracked, but it's usually the brittle rotor.
4. If it lights and shuts off, it's a flame sensor.
Like I said, there's nothing to them. Parts are MOSTLY the same, with some slight variations.