Hi Chris,
old plug in the decomp hole should work fine unless it has a bit too much compression, I used one this way for about 10 years till it got to a stage where I couldn't find compression. I just found that motor a couple of days ago sitting on the shelf, I must have had that for over 40 years, might chuck a set of rings in it and fire it up again.
No spark could also be condenser problem but another plug is best option to check, any plug will do to test for spark. You don't need the plug to test for spark, just hold the end of the plug wire 1/4 -1/2 inch from somewhere on the head and pull it over, good spark should jump 20mm
Not sure what you have done with the filter looks like the throat is still open or is that the sponge for the bottom of the filter
