High elevation non- pv Burnham boiler with Tejurland powerventer installed at sea level
I'm not clear on what you need repair help or a help deciding how to deal with the situation. Any burner, be it oil, propane or whatever at high elevation needs to be derated. So for gas, smaller orfus sizes. So if the boiler was designed for 2000 foot elevation it should be under firing at sea level. But in this case with your 203ncl-tei2 it's up to 2000 feet elevation most gas burners are factory 0-2000 feet from the factory including this one.

Boiler information 

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The guys at Burnham are telling you that it is not approved for power vent. That just means that it was not tested for the application, which means it's not to code, no matter what. That also means you are liable for any harm or damage if you are aware of the issue. That doesn't explicitly exclude a aftermarket add-on power ventor. The aftermarket manufacturer can give approval for the specific model and assume liability, but you would need documentation. If I'm ever in that situation I will inform the customer and depending on my gut feel about them (or how well I know them) I will red tag it and give them a couple of options for replacement or do the repair and tell them that I was never there. I would only do the latter if I really know them well and I'm confident that it's safe. So basically, tag it. If the motor is humming and has the right voltage and the cap (if equiped) is good it's the motor. But I wouldn't change it. I'm in Canada and our codes for HVAC are more strict than in the US so it's hard for me to say to you that you should decommission the unit in the middle of winter but if the s*** hits the fan you don't want to be holding the bag.