Please note. None of this information originates from me. Acknowledgement made to Dermot Wrycraft.

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The thermistor, if that's what it is, is there to linearise the meter for temperature, hopefully.

Likely Faults etc.
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If the polarised capacitor fails, which is quite likely, the meter will probably go erratic
(low) followed by total failure. It can be replaced by a modern equivalent, e.g. Tantalum.
If the transistor fails the effect would be similar, but it appears that the transistor
is in fact pretty tough!
The resistors won't fail.
The coil/transformer won't fail unless damaged/hit.
The meter won't fail unless mechanically damaged, or damp gets in.
PCB hairline cracks are possible, can be fixed by "tinning" the tracks.
PCB earthing depends on one of the mounting screws (not just the one which is on the outside
case) and should be checked.

I have not found a direct equivalent for the transistor. However any generally similar
germanium PNP device should do. Silicon devices probably won't do even if PNP.

Comments
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You can see that a revcounter like this won't work with 'clever' ignition systems like CDI,
and in fact, depending on what kind of "coil primary current" flows, it could be damaged.
However, simple transistor switch devices which replace the points should not damage an RVI,
though their faster/better switching may encourage e.g. capacitor failure.

Please Note. I cannot remember where I found this picture. If it's yours let me know and I will acknowledge with your name.