Determining Suitable Replacement Transistor?
Summarised_solution:


General purpose high current, high gain SOT23 bipolar &amp; MOSFET transistors are available at modest cost. These will work if the part is used as an on/off saturated switch and will probably work in other roles. 
Required polarity can be determined by a single measurement.




SOT23 small signal transistor is usually -
 b c e for bipolar
 g d s  for MOSFET

as per diagram below.



Measure voltage 2 to 3.
 If 2 is +ve wrt 3 then = npn or n channel. 
 If reversed then pnp or p channel.

A high current high beta npn or pnp will work in most bipolar cases.
 BC817-30 / BC807-40 are good.
 -40 means AVERAG current gan of 400. ie 250-600 range.
 50V, 800 mA rated for thisFairchild example sold by Digikey
 Rbase of 1k to 10k should be OK.

MOSFET N Channel needs low Vgsth.
 CES2310 utterly superb if you can get them.
OR

36 cents /1 in stok at Digikey for this
 2.6v Vgsth , 60V, 1.A continuous
 IRLML2060

But, Better! -->

INFINEON BSS205 N !!!
   20v, 2.5A, &lt; 100 mohm Rdson at 2.5V Vgs.
 In stock Digikey  37 CENTS /1

No gate resistor needed. Use say 1K if desired.

For P Channel use Digikey selector guide to select out specs of chouce then sort by price ascending in quantity 1.

This is a truly marvellous part for amateur general use.
 Be ware of 20V Vds max but otherwise marvellous.
  eg 0.2V Vds at 1.5A at 2V Vgs.
 500 mW disispation and 250 K/W mean due care needed thermally (as in all SOT23)
 BUT very low Rdson will keep it OK in many cases.
 eg 98% of these have &lt; 80 milliohm Rdson at high temperature.
 At say 1A dissipation &lt;= 80 mW for 20 C case rise on modest copper. 



If you MUST have 30V there is the very nice 41cents/1 
IRLML6346