Ok, the Winegard dishes got F/D of 0.275 approx.
There is no feed these days that comply to these specs except Seavey Engineering.
When these dishes were sold in the 80's they always, those i sold anyway, came with a Seavey feed with a C120 hole or a Seavey polarizer model PR-120 which included the feed to bolt to the buttonhook.
If you can find or can afford a Seavey feedhorn to your dish it will deliver one of the best performances of any dish in that size.
The deep design was excellent as it shielded the LNB from remote noise at the sides.....
As i said again a LNBF with integrated feed as shown in your picture won't work good at all, they are designed for Offset dishes with F/D of 0.60 approx.
You will have huge signal spill with a LNBF.
Just get a good C120 LNB like Invacom, Inverto or MTI and buy the Seavey feed or polarizer and you will be set.
Please check the dish for warpage before buying this as if the dish is warped you will never be able to correct it again to it's true parabolic shape.