If you're using an airswitch for time counting, which is perfectly fine, you want it to be calibrated so that it is right on flying speed (ie probably somewhere just above stall in the landing configuration), any ultralight is going to be flying well under 60. It's wheels up, to wheels down, 60 is more like "time in cruise" for most ultralights, that's not acceptable.
An airswitch is quite a valid means of judging service life, because...
1. Under flying speed, engines are not likely working hard, if you idle an engine for 1500 hrs, you are likely not putting nearly as much wear on it as if you run it flat out for 1500, and the TBO estimate needs to be much closer to flat out than idle.
2. Manufacturers, unless otherwise noted in the documentation, account for this in the service life estimates, because;
3. As others have pointed out, thats what the rule makers around the world reckon it should be based on.
It is important to remember that any manufacturers service life number is an estimate at the best of times, we hope a pretty conservative one (and this is usually true, evidenced by those who run on-condition far in excess of published service life).
As for the "Overhaul", it comes down to if the mechanic who did it has "zero timed" it in the logbook, when they want it to be overhauled next, is it at 1650 hours total life or 3000.
If it's not logged, then 1. find out why the hell not, and 2. treat it as if it never happened until you get confirmation from the engineer who did the "overhaul" as to if it was zero time or not.
That said, replacing NOTHING at 1500 and zero timing... very unlikely. Replacing nothing and running on-condition for another 150 hrs... closer to reality.