It sounds as if the console might need replacing. But that diagnosis is not certain.
Since my own resources have to some extent failed in this case, and my diagnosis is (of necessity I think) failed, I have only two courses left in all fairness:
1. I can (if you will tell me your region, by zip code or by nearby town/city and state) research nearby repair co.s working on SportsArt treadmills, that you might contact. An experienced repair-person might be needed just to get a right diagnosis of this problem.
2. I could Opt Out of this case, and hopefully a treadmill Expert here with more to offer than what I have already said, might pick up the case.
Let me know which (or, both) of these you would prefer.
Hi, after you lost the red stop cap, did the unit still work correctly? If not this may be all you need to get it running which is the safety key or magnet.
If it still had worked fine after losing that cap, then it sounds like your console has gone bad and needs to be replaced, but since its such an old machine it doesn't look like parts are even available.. Most treadmills only last about 8-10 years regardless of use so it may be time for an upgrade unfortunately . If its just a safety key issue let me know and we may be able to work around it to get it running again.