+1 I know when I had an issue, it was definitely
me that was the cause.
Not sure what you mean by compress the cylinder or why you need to replace the spring and lifter. Is there other missing or broken parts?
In order to put the rocker arm back on you would need to:
1. Ensure that cylinder is top dead center (TDC)
2. Keep that cylinder at TDC using air or rope or the method of your choosing
2a. Now that the valve stem can't fall down into the cylinder
3. Compress the Valve Spring down using something like AGA's N62 valve stem seal tool kit, just enough to give you room for step 4
4. Using only your hand/s, place Rocker Arm back on the lifter and valve stem correctly
5. Decompress the Valve Spring
6. Drink beer
This assumes nothing else is wrong with that engine and the parts involved are all in good shape.
I'm certainly no expert, but I would guess it's certainly related. Whatever caused the rocker arm to come off in the first place is probably the cause.
