Firstly, welcome to the Forum.
Are these ignition modules interchangeable?
Depends. It's unwise to assume that one maker's 'box' can be connected to another maker's 'trigger unit' and they'll automatically work together. However, particularly in the case of the Boyer
-Bransden, Sparx and Pazon units:-
. If your Sparx e.i. was one of the original batch, it was a licensed copy of the Boyer
-Bransden Mk.3. How much the current Sparx e.i. owes to that, I couldn't say, and even parts from the two Sparx systems might not be interchangeable - try Andy Gregory
for spares/ more information?
. The couple that run Pazon used to work for Bransden, he might have given them permission to use the same trigger unit design?