rgehrig,
This site might be of interest to you.
http://www.qldrifle.com/shop-q-storere "inserts" Measure the internal diameter of the tube in millimetres. That will give you the tunnel-size of the fore sight elements.
"mounting the stock"
No idea how your Omark is bedded. One of mine had the rear bolt in front of the trigger but that was a custom job, performed by a 'smith. The action from the trigger back, simply floated. Standard position was in the tang. Often, in custom jobs, the safety was deleted. In others, pillars were added.
Your poor Central is in bad nick. I have never seen rust like that. There should be experts here who can tell you how to proceed but above all, don't wind the knobs in case the rust enters the elevation and windage nuts. In the meantime, hose it with G96 and just let it soak.
Fore sight tunnel
I am wondering how good the tunnel looks through the peep. Does the "knurled edge" of the tube that is screwed in to the tunnel-mount, look precise enough through the peep? In some cases it might be sharp, depending on your eyesight. Mine had a "plain" circumference and was a bit fuzzy but speaking for myself only, I think yours might give me a problem with centreing it in the peep. I keep wondering if the front and rear tubes should be exchanged, one for the other: swapped around, that is
If you are not experienced, choosing a fore sight element will be a Herculean task, especially if you need an Eagle Eye and a corrective lens, like I did but Good Luck and let us know how you go.