You can get a Wilson Drop-in beavertail and it will drop in. The catch is, the tail itself is not a close fit to the frame. There is a gap Which you may or may not find cosmetically objectionable. I didn't mind, and it feels comfortable.
I tried to swap out a CMC safety on my mil-spec and it had 2 clicks, rather than ON and OFF. It stopped just low enough to let the hammer drop, but not enough that the slide could have moved. Had to stone the lug, file the plunger engagement surface and make a new dimple. Not an easy part to fit. Don't know about other brands.
I installed a King's drop in beavertail on my mil spec. This was after installing a Nowlin trigger kit to get rid of the trigger creep that came with the gun.
The King's drop in was a true "drop in" part. The picture below is how it ended up. The small gap is what they are talking about. For me, I can live with the gap. The cost of the Kings unit was $36, so for that price, I was happy and it works great.
I also installed a Smith & Alexander flat mainspring housing. This got rid of the factory ILS. Ambi safey's dont do anything for me, so I left the factory one installed.
One of the best additions I did for accuracy was install an EWG Bushing. This made a world of difference in consistence accuracy at 25 yards. Next to the Nowlin trigger, it was the best $30 I spent.