I understand this to be a dead engine compression test has been conducted here and that no. one cyl. Is lower than the rest and you want to determine why?
My take would be the first check here is to check the tappet clearances on no. one cyl.
A tappet with no clearance or too tight a clearance would obviously produce a low bar/psi readings as it's not airtight.
To determine where the leakage is exactly?
Many, many years ago I made up a tool to locate where any bar/psi leakage might be.
Tapping off a airline via a pressure regulator and into a spare injector body with the pintle needle removed to allow an unrestricted airflow supply you tap or weld/braze a PCL fitting into the injector body this being for the purpose of instant easy access.
Attaching an airline hose ( good for 200 psi at least ) you bolt your modified injector back into the head and start off with 50 psi winding it up to over 150 psi with all tappets rocking on TDC.
Watch the gauge, any drop off in psi is going somewhere.
If the needle does not remain constant off with the air induction system from the manifold and listen closely to see if any compressed air is being passed.
If it is then suspect inlet valve not seating properly.
Ditto the exhaust manifold system, coolant system ( for cracked head ) although a cracked head into the waterway would pump up the coolant system resulting in an obvious overheat system normally.
But the crack a lot of times cannot be seen until all valves are out and is not always into the water jacket.
The only other way for your leakage test psi to go is down past the rings into the lower half of the engine, in a lot of cases into the sump.
Again access required here via any openings i.e. breathers, pipes or dipsticks present and listen for any compressed air being passed.
Your problem here might be the surcharge on the old injector ( which sometimes is very heavy on exchange units ) and/or workshop facilities available to you at this time.
If you are unsure about any readings whip another cyl. injector out and compare readings taken.
That's how I would tackle it anyway, best of luck and do let us know the outcome as it all furthers our interest/ education in to the subject of diagnostics.
I would hate to be in a position whereby owing to lack of made up test equipment you had to sub the job out as to me that removes a lot of challenge/ interest in it all.
Don't sail off over the horizon without letting us all know what you find mind.
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.