Nothing is illegal for you to USE on the ham bands as long as it doesn't have too much splatter (in band spurious emissions) and it's clean out of band (harmonic suppression, etc)
Now I think there's a good chance that it may be tough to get a "Superstar 3900 Turbo ZX" to put out a particularly clean signal on 10m SSB, but if you are careful with the mic gain, take advice from other hams on how you sound and whether you're splattering, and don't worry if you can't get the rated power out of it and still have a clean non splattering signal, it will probably be okay.
A good strategy might be to ask very loud hams to check your signal for wide splatter without telling them the name or type of the radio
. Just tell them you want to make sure you've got your audio adjusted for a good clean signal and ask them to check a few kHz up and down for splatter.
If people always give you bad audio reports or tell you you're splattering without you asking, just junk the thing. If they tell you you have a nice signal, that's 90% of the way to legal operation as long as the rig has adequate harmonic filtering, which it probably (?) does.
As far as un-doing the "CB mod" don't worry about it. It's legal to OWN a rig that transmits on CB or other non-ham frequencies as long as you don't transmit on CB or other non-ham frequencies.
I will be honest with you, there is a very good chance that your Superstar is pretty much useless for ham communications on 10m. It seems like it's channelized which may make it hard to get on others' frequencies. If it doesn't have a frequency counter make absolutely sure you are on an allowed SSB frequency before you call CQ
If people keep telling you you're off frequency even though they sound on-frequency to you, junk it.
It may be better just to use a legal radio like the Icom 706 or something similar,
It's better in dozens of ways to use an actual ham radio instead of a "10m" radio that's actually manufactured specifically to be purchased by illegal CB and freeband ops. That doesn't mean you can't use the latter in some cases if you're careful
. But it is worth recognizing that these CB/Export type rigs have a lot of deficiencies for PROPER ham use even if they're technically legal. The thing about ham radio is that we have the freedom to use any old thing to transmit on the ham bands, but that comes with a responsibility to ensure that we have good signals that stay within the ham bands and furthermore don't take up excessive bandwidth within the ham bands.
That responsibility often means it's not worth putting some of these CB/Export rigs on the air, and certainly means it's not worth spending money on them but if it's all you've got and you're careful about making sure it's not annoying others with a bad signal, it's okay. Again, do NOT under ANY CIRCUMSTANCES say the name of the radio... it will greatly bias the reports
If people tell you it sounds bad without knowing what it is, toss it ASAP and find a better rig.