To answer these questions, it is unlikely that you will find a unit that has not been reset at least once regardless if a tag is present, they are out there, mostly ones that spent their entire lives in storage (prepositioned resupply for a war in Europe, etc.) However the MEP-002a and MEP-003a's are powered by a militarized version of the Onan DJB and DJC engine, production of these engines stopped around 1994 with very few produced the last couple years, so even if you get a "new" never reset one these are likely to be 20+ years old and may be as much as 40 years old so in need of some parts due to dry rot. There are also a few units with build dates around 2001-2006 there being built by various contractors from spare parts. Having said that a reset unit should be considered almost as good as a fresh factory unit, the reset involves a complete engine rebuild, including resleeving the cylinders and recertification of all components.
I have a MEP-002a and a MEP-003a bough from GL, both are a complete mystery to me on history, the 1980 build date MEP-002a which showed about 2000 hours on the meter was stenciled with "Major Service" dated in 2008, your guess is as good as mine on what that means, however I suspect it had not been ran since that "Major Service" work as it has a brand new fuel tank, new fuel pumps, new hoses, and what looked like a new starter on it (new enough to have a label with a web address), and all fresh filters, it did however have a problem with the voltage regulator that I ended up solving by swapping in a known good circuit board after unsuccessfully trying to repair it with new transistors. (it would run good for a couple of minutes then the voltage would spike). This MEP-002a otherwise runs great, and was fully functional until the frequency meter went out on it a few months ago, it however is a bit of an ugly duckling as the CARC camo paint job was brushed not sprayed on.
My MEP-003a is another real mystery with about 850 hours showing on the meter, it too had voltage issues when I bought it, in this case the problem took a fair amount of hair pulling to track down, it seems someone had installed one of the control transformers backwards/upside down and it had fried. (a cheap enough fix once the problem was found), and I am still working getting the engine to run right and pull full load, but I am getting there. This generator lacks many of the markings and data plates we expect to see, and is painted plain green CARC. Even the expected Diesel Fuel Only stencil is missing. The only markings were a unit marking and Generator #3, the little reference I can find online about the unit implies it belonged to a special Air National Guard rapid deployment unit that seemed to specialize in deploying after natural disasters. I suspect this one sat up for a while as I think the unit it belonged to was disbanded about 10 years ago, which may account for the engine issues.
As to your question about parts, there are some you might still get from Onan, but expect them to be expensive and in limited supply, they seem to be doing their best the last few years to kill off the parts supply for the older J series generators so they can sell new generators in their place. The J series engine was a real work horse, and many J series civilian generators are still in service much to Onan's current managements dislike.
Ike
