Is it possible to replace a DIAC circuit with an SCR circuit to drive a TRIAC
The circuit will quite likely work without the DIAC - i.e., short out where the diac should be until you can get one - but may be a bit "twitchy".

Without the DIAC the TRIAC will turn on when the trigger voltage gets high enough. The problem is that this is not very well defined and will vary depending which quadrant the triac is operating in and, probably, with temperature. 



Figure 1. Trigger angle and resultant waveform.



Figure 2. Triac triggering modes and quadrant numbers. Source Wikipedia (with addition of numbers).

Note that your circuit works on quadrants 1 (both mains and trigger positive) and quadrant 3 (both negative) only.

If the triac triggers levels in the first and third quadrant then there may be a noticable flicker between positive and negative half-cycles particularly on the  romantic end of the pot.

The DIAC solves this by holding back the trigger until the capacitor voltage rises to about 22 V (if my memory is right) and then allows the capacitor to discharge through the gate pin. The point is that a fraction of a volt difference on 22 V will result in less phase angle difference between positive and negative half-cycles than directly triggering. The downside with the triac is that you can never get 100% power.