Trust me, don't waste your time or money.
You do know the Mk20 comes in 6.5CM? If it helps it uses the same mag geometry as FAL only with an AR type latch and cutout. It also doesn't have the stupid step down the 7.62 version has. Proven Outfitters is taking orders for all versions I think and has .mil/vet discounts, but I got my 7.62 for five hundred less on GB of all places last summer.
Why the fuck you'd invest in an FAL when that exists is beyond me. I finally unloaded all my FAL stuff, basically paid for the Mk20, ten mags, bipod and monopod. Essentially a one for one trade. (It was a NICE FAL but still an FAL).
Find a new/almost new Austrian para with a Steyr hammer forged, chrome lined barrel. Leave it stock. Get a few mags, you're done with FAL's. Doesn't need an optic, doesn't need upgrades. It's not a suitable DMR. BTDT. I'd probably still have one if I hadn't done all that shit to it.
If you gotta be fancy, get a folder and a DSA scope mount with the cutout for an ACOG base. It'll still be too high for a cheekweld but it's the best you'll get for an FAL. That or an LED trilux if you can find one.