Lester is right in recommending concertina.net. However, many of us melodeonists also play concertina, so we can have some input and opinions on your query.
A Stagi concertina is not the best make and can be marred by poor quality components and workmanship. However, just about any instrument can be made to sound better by having the tuning corrected if it has drifted somewhat over the years. So it might be worth having the reed response optimised and tuning checked and corrected where necessary. Stagis are hybrid concertinas with accordion reeds. That's not necessarily bad - there are some excellent hybrid instruments available. But it will never have that quality of sound produced by true concertina reeds.
I'm not sure what you mean by 'having the bellows tightened'. Is there an air leak which needs repairing?
If you are getting to a reasonable stage of competence in your playing, your Stagi will probably only hold you back from further improvement. You don't say where you are located but the best thing would be to try as many other makes of concertina as you can if at all possible.
Of the hybrids, you might want to consider these makes, for example: Marcus, Morse, Tedrow, A.C.Norman, Wakker 'Clover'.
Traditional-reeded concertinas tend to be very expensive, but it's possible to get a good vintage Lachenal or Jones without necessarily taking out a 2nd mortgage.