Welcome Orion and well done so far.
Please dont be offended byh anything I mention it is given as ideas not criticism.
You mentioned that you stripped and cleaned the injectors. This is normally a job that requires specialty equipment when assembling and setting the working pressues, so this thought should go on a list of why it may not start.
In order to bleed the fuel system it is best to do it in stages. May sound silly but confirm you have fuel running freely out of the tank. Easiest way is to loosen the fuel line at the filter body if this is the 1st stage of fuel flow. Next bleed the filter housing. There should be a bleed screw on top of the housing. Next I guess the fuel goes to the 1st of the fuel pumps, ensure fuel runs freely to each pump in order and make sure there are no bubbles. So far this may be all gravity fed, if not you are going to get tired of pumping by hand.
Next with the throttle open wide start turning the engine overand see the small amount of fuel rise from the top of the pump. It is really just a dribble at this stage. Do this for all 3 pumps. Then disconnect the fuel line from all the injectors and crank again until you see the same amount of fuel come out of the fuel line with no bubbles, again on all 3 lines. Connect the fuel lines and attempt a start up but be ready to shut it down just incase.
This is how I bleed a diesel fuel system only mine are single cylinders and I have been able with one engine to remove the injector and connect it to the fuel line, crank it over and physically see the atomised spray. If you see this spray do not try to touch it as it is strong enough to impregnate into your skin and this is painful and dangerous to your health.
As I have never had to do spill timing I am unable to help but ajaffa1 - Bob, can help.
I found my timing marks on the outside flat section of the flywheel which showed a - mark for TDC and a lightning bolt for spark/spill timing I think. You will need to find TDC on compression then really clean up the rim of the flyweel at the 12 o'clock position and use a good light to help you see it. Once I found mine I really cleaned them up with a wire brush and filled the marks up using a white oil base marker pen.
Good luck and keep us informed.

Welcome to the forum Orion, first question: was this an SR3 or an SR3W (water cooled)? Second question, when a cam shaft breaks it usually does irreversible damage, what where the bits you found in the sump, did you replace the camshaft, the bushes what about the cam followers, did someone line bore the new bushes? Not an easy task for a non mechanic.
Assuming you have had all the above done correctly did you number the injector pumps and the shims that sit beneath them? If not you are going to have to spill time each injector pump to ensure it injects fuel into it`s cylinder at the right time. Then you can think about linking the injectors together and bleeding the fuel injectors.

One hell of a project you have taken on, hat off to you Sir!

Bob