By all means try the dizzy you have and see if it "feels right".  Pay careful attention to any noises such as knocking or pinging that indicate either too much advance or fuel of too low an octane rating. 

If you really want to know what you have, you can do a semi-accurate measurement of the curve.  To do this you will need an advance type (dial back) timing light and a tachometer.  Warm the engine.  Disconnect and plug any vacuum advance tube that connects to the distributor.  Measure the advance at idle noting the degrees of crank advance and the engine RPM.  Use the idle speed screw to raise the engine speed in 200 RPM increments.  At each increment, measure the advance.  Do this from idle to about 3500 RPM.  Repeat the process about 4 times and average your results.  Then enter your data into a program like Excel to create a scatter chart with connected dots.  You'll end up with an advance curve map for your particular distributor.  You could then email that to SimonBBC and see if they can better identify what you have.

Doug, thank you! You were correct—it is a Powerspark 45D. That helps narrow it down some. I called Simonbbc today and it looks as if that is as far as the trail is going to lead unfortunately. There are at least 4 different electronic ignition 45Ds that they make ("regular", high energy, 998 specific curve, 1275 specific curve), but they have no way to tell which one it is. They suggested that if the car starts up with it installed then it is the "right one for the car". They were very polite and friendly, but not terribly helpful. It will be good enough for now, but I will probably move this to the backup / spare parts bin and get a different dizzie for the rebuild.

Thanks again!

I don't recognize the brand of module shown in your pictures.  All the Pertronix modules I have seen (original, II, and III) have the wires potted into the module... not terminated with spade lugs.  Therefore, not likely to be theirs.  

The dizzy casting appears to be a 45D, not 25D.  The 25D has a "rounded" bottom and LONG clips to hold the cap.  The 45D is squared off on the bottom (like yours) and has short clips for the cap.

EDIT:  To me it looks like a PowerMax module installed in a reproduction 45D.  See the first picture in the link below.
https://sites.google.com/site/electronicignitionreliant/-3-things-to-check-do

Looks like a Pertronix Distrubutor..

looks like a normal 25d Drive end (narrow fin) that goes into the distrubutor drive
and judging by the 'finish' of the outside alum casing  same' colored vac advance + clips.

as seen here. =

https://www.ebay.com/itm/Pertronix-Distributor-Side-Exit-Austin-MG-Morris-4cyl-w-Lucas-23D-25D-45D-12V/170915969414?epid=214962226&hash=item27cb62b986:m:mfeWUHdmYclfbHPziyxhIzA:rk:4f:1&frcectupt=true&autorefresh=true

The spare engine I recently bought came with an electronic dizzie that has no markings on it anywhere (that I can find). Can anyone help me identify what kind it is? 

Thanks!
Scott