Wasn't it because Ford was sick?
Yes, Ford supposedly improvised in order to cut the whole scene of a demanding sword vs whipfight (or something on that order) short.
Another ingenious Ford impro is the "I know" reply to "I love you" in Empire Strikes Back. As the saying goes, due to fatigue after plenty of repeats on that scene.
IIRC, Christopher Walken was considered as Han Solo. Weird, but could have been awesome.
On Indy...I have absolutely no idea who could replace Ford (without refering to Selleck- according to IMDB, while Magnum was the reason Selleck declined, it turned out afterwards that he could have shot Raiders in time before filming of Magnum started). Everyone coming to mind is just too young back in 1981.
Again quoting from IMDB:
Actors considered for the role of Indiana Jones included Nick Nolte, Steve Martin (who chose to do Pennies from Heaven instead), Bill Murray (who dropped out due to scheduling conflicts with Saturday Night Live), Chevy Chase, Tim Matheson, Nick Mancuso, Peter Coyote, and Jack Nicholson. Harrison Ford was cast less than three weeks before principal photography began.
To me, a load of strange choices. Steve Martin might have been interesting, think of "Dead Man Don't Wear Plaid".
I do not find speculation on the role of Deckard that intriguiging, because to me, Ford didn't "define" that character as much as Han Solo or Indy. IMHO, anyone who could play a cop could play Deckard.