Hi mememe - interesting problem. In the firing sequence plug no.1 follows plug no.4 (1-3-2-4), so you may be igniting fuel in no.1 on its induction stroke. There certainly seems to be a timing difference between the cams and the distributor. The vacuum canister bodge may be something to do with it, but if the distributor plate (the bit that moves with applied vacuum - the old "points plate" in fact) is in the same position as before, and is not jamming in any way, I can't see how it can be responsible.
So my only other suggestion would be to recheck the cam timing to confirm that both cams are accurately lined up with the timing marks at TDC (one might be a tooth out), and if that is OK, carefully check the marks on the flywheel - some engines only show TDC and two intermediate advance points and no idle timing mark, on the assumption that you will be using strobe timing to set everything up.
I won't say anything about dropped valves yet, you don't want to consider that until you've double checked everything else. Hopefully its just a timing issue.
My boxer engine doesn't have an idle timing mark, so when I set up what I thought was 8° BTDC at idle, it was in fact something like 20° BTDC, which could make all the difference!
Good luck. If all else fails, go back to the last known good configuration and start again. If you suspect valve damage, do a compression test on each cylinder - if one is markedly down on the other three, that might just be down to poor valve timing, or you might need to get your chequebook out...