man, i wish i had recycling centres like you have all we have is boring furniture and old things.
the Roma has no steam control block or any plumbing leading to and from the steam arm, it's driven straight off the thermoblock so the problem will lie in either a faulty steam on signal from the control board, blocked, dirty, broken piping between the pump and thermoblock or in the thermoblock internals or a blocked/dirty steam arm.
seeing as you said it seems to be trying to pump steam, it would appear that the steam on signal for the control board is working.
if the o-ring is cracked or worn, the steam function will still work but it'll just leak steam and water out of the top. i doubt the o-ring is the main concern here.
have you checked that the wand tip isn't clogged with old milk and preventing any steam from getting out? if it's clogged, you'll hear it try to work but no steam will come out, or a tiny bit will just hiss out. i know it's simple but sometimes the simplest of things evade our thoughts.
i'd say that or a blockage further up the arm is the cause.
for a quick narrowing down of where the problem lies, remove the steam arm from the thermoblock and activate the steam. if it then pumps steam properly (just out of the thermoblock, be careful cos it'll probably go everywhere) then it's a problem in the arm, if it still doesn't work then it's a problem in the thermoblock/piping.
like you said, do a full clean and flush of the thermoblock first and clean the steam arm and see how you go.