It is to be used in an analogue oscilloscope vertical and horizontal amplifier sections and also trigger sections. If a faster transistor is to be used I MAY be able to make this a little bit faster oscilloscope than 5MHz.
In this case, you should check the actual voltage and current rating required by the respective circuit and try a suitable faster transistor. Speed is a matter of all circuit parameters, not only transistor fT and capacitances. Furthermore placing a high speed transistor in a circuit not prepared for it may bring up oscillations.
I agree. The bandwidth limitation would be mainly located in the output amplifier that has to be possibly redesigned for a profound improvement. As a first step, you would want to get an overview of the present circuit's behaviour, either by a simulation, or even better, by a measurement.