Need a little help with a Chinese tesla coil
The BD243 transistor is chosen carefully for this application of boosting voltage by driving the collector hard with gain.  The product of the gain and max current or \$\dfrac{Ic^2_{max}}{I_b}=h_{FE_{Min}}*I_c=15*6=90A\$  for BD243
(This is just a Figure of Merit FOM, not actual current )
THe other critical metrics or FOMs are; \$R_{ce} , f_t, V_{ceo}, Pd\$

The Rce=Vce(sat)/Isat of the transistor and supply voltage is what limits the Pd=Vce(sat)²/Rce  @ Ic when the base is driven hard (Ic/hFE(min) to saturate. Consult graphs in datasheets for these.

Any replacement ought to meet or exceed these FOM's

Stock and cheap at Digikey

Match these characterisitics from this list for above product and Vceo or buy the same or better part.  The Ft toggle speed is also critical as the rate of change of current is a gain factor for producing voltage. V=LdI/dt

If the part doesn't get warm, you can always raise the supply voltage carefully.