In my home country they say "Cheap as Onions" (eq in English : Cheap as Dirt)

Looking at the prices on Onions nowadays ...

A stitch in time saves nine.

This refers to a time when clothes were hard to come by and often homemade. If they tore, a small tear could be fixed with one stitch of needle and thread, but if let go too long, it would take many more stitches to repair a bigger rip.

Change your oil (or tires) in time, or change your car.

went from hell and back, obviously it has a very christiany origin to it as is moving heaven and earth.

"pot calling the kettle steel" just doesn't have the same ring to it.

Eating a dead horse - not unless you buy the budget beef mince at Tesco

Read like rabbits - rabbits can't read or even hold books

Nip it in the butt - that's sexual assault, you're not allowed to do that any more

Get down off your high horse - you're not allowed to get animals high either

Kids should be shorn and not herd - goats have hair, not wool, and can't be shorn. Their natural tendency to herd is a difficult behaviour to break.

Under the weather - unless you're a pilot you're going to be under the weather most of the time

A blessing in the skies - unless your bishop is a pilot too, that's not going to happen.

Fight off more than you can chew - I don't know where to start with this one,

Oh, man, when it comes to nautical idioms, I could go on all day. There are so many, and they've lost their literal connections so thoroughly, you probably don't even think of some of them as idioms. A few that occur to me immediately:

• taken aback (wind on the wrong side of the sails)
• give a wide berth (extra space for a vessel to maneuver)
• leeway (unwanted downwind slippage of the vessel when in motion)
• above board (on deck, the board being the section of the hull running fore and aft just above the water line)
• onboard (to load onto the deck of the ship)
• go by the board (to fall off of the ship into the water, passing the board on the way)
• hand over fist (two-handed motion to pull in sails, ropes, etc. as quickly as possible)
• high and dry (above the tide line)
• loose cannon (a gun insufficiently secured would cause havoc on a warship when fired, or as the ship rolled in waves)

Honestly, I'm not sure that I want modern equivalents, since many are so rich in imagery.

I don't think it's the char/open fire but rather that both pot a d kettle would be made of the same black cast iron

I love head over heels as, in love.

It's vice versa that makes actual sense, but that was considered too obviously risqué in the age the phrase was coined.

"Avoiding things like the plague" because people did not actually seem to want to avoid the plague last time around.

All the horse ones because we don't use horses anymore. There a ton of them.

Don't look a gift horse in the mouth. Who tf gets a gift horse?

Saddle up. Who tf has a saddle?

Hold your horses. Who tf has horses to hold?

Get off your high horse. Who tf has a high horse?

Beat a dead horse. WTF.

Not quite updating, but in my house we say "like sweeping the yard" to indicate that something is pointless and wasting time.

"Boss has us doing obviously pointless busy work today"

"Yeah, next he'll be having us sweep the yard!"

Or derogatorily:

"I want you to go do that thing."

"Sure, you want me sweep the yard when I'm done?"

I always thought of that idiom as the pot (something generally cast iron and not polished) calling the kettle (something that is polished and therefore reflective) black.

Being that the pot sees itself in the reflection, but calls the kettle that as a derogatory thing, unknowing of it's personal alignment.

whats the point of a modern replacement? idioms connect us across time and when they really have not modern sense (which there is enough understanding of cookware I don't think we hit that with pot and kettle) then it becomes an interesting backstory.

I assumed the idiom was from a time when all cookware was made of cast iron.

Don't put the cart before the horse.
Don't submit your post before you write it.

"Have your cake and eat it too" doesn't really make sense. Cake is not rare or expensive enough that simply possessing it feels good. If you have cake, all you want is to eat it.

The italian version, "you want a full barrel and drunk wife" illustrates the same situation much more clearly.