+1 on FB's suggestions.

If you have a stripped bolt, it's not hard to simply find a replacement that should fit and have the correct length, or if you notice that the spacers stack barely clears the top of the steerer, then adding a 2mm spacer shouldn't look too hokey, and then you can kept star-nut and bolt as is and tighten using threads that aren't stripped.

If star-nut and bolt are both compromised, pushing the existing star-nut down is the exact solution I use too. I gotta bag of these, and the main reason, I think, is because half the folks who service these bikes seem to have a bad habit of over torquing the top bolt -after- they've already adjusted the headset for play, or they simply don't leave enough spacers and are tightening against the steerer tube.

If you're missing the centering cone for the headset, well, you may need to get one to replace it. That may mean contacting manufacturer or having the LBS do that for you and order the part. The headset shouldn't be ridden without this.

If you need to ride the bike now and you're not missing the cone, it may be possible to simply loosen the stem bolts and then firmly press down on the stem near the steerer tube and then tighten up. There may be some play, but if you have a strong enough person to manually "squeeze" the headset per se, then you may be able to snug up a stem bolt just tight enough to prevent slip, then relax and complete the tightening process for both stem bolts to secure the headset in place. The tightness may actually be pretty close to using the top bolt/cap to do the preload. So this will give you time to source replacement parts.