Q:

BC331 and BC541?

I tried to find datasheets for BC331 and BC541 to compare them with BC547.
Can I replace BC331 / BC541 with BC547?

A:

BC331
NPN, 60v, 30 mA, hFE= 125, Ft = 100 MHz, TI, low noise, TO92
 Very old and tired.
Some data here
Related selection guide here
BC541: Does not seem to be a known device.

A BC547 will be a good substitute for most older silicon NPN transistors.
 Note that BC541 = 60V rated and BC547 = 50V. This is extremely unlikely to cause a problem.
For most small signal transistor purposes I use and recommend the following:
BC337-40  NPN through hole Datasheet here
 BC327-40 PNP through hole  Datasheet here
 BC807-40  PNP SMD Datasheet here
 BC815-40 NPN SMD Datasheet here
All 45V, typically 0.8A (varies slightly with manufacturer), TO92 or SOT23
 Ft = 100 Mhz
 Hfe (current gain = 250-600, mean = 400.
All these are often available as cheaply as any other transistor available (look around) and have abpout as good a combination of current, current gain and general usefulness as anyhing else going. When you find better cheaper please advise.

