With a wet leather ball and leather boot, how would players of today compare with those of the twenty and thirties?
No comparison. Players of today in the top leagues are freakish thoroughbred flat racing horses, compared to the working beasts of 100 years ago. For skill and speed, none of those 1920's players could compete in a modern professional league match. Non-league, yes - 7th or 8th tier or below perhaps. Football has not changed so much at that level. But players of today would not last 5 minutes in a 1920's league game either. Back in the 1920's Charlton regularly played games on consecutive days (Saturday & Sunday for instance). Several times we played three competitive games in four days - often with the same 11 players too. We used to play on Christmas Day and Boxing Day every season - home and away against the same opponents. No modern player could live with that schedule. Neither could they play with 1920's kit, boots or the 1920's ball, as you have said. Footballers 100 years ago were a much tougher breed of men. No substitutions, only rudimentary treatment for nasty injuries, playing in all conditions on pitches that were more like battlefields. Hardly any protection from the strongest tackles. They were hard blokes back then, and I daresay more honest and sporting too. Today's players are melting snowflakes by comparison. They are faster and trickier, but infinitely more fragile and lightweight.
It would be like landing on a different planet. Not only did the players have a different mentality so did the fans although my first game was as recently as 1956. There was rapport between fans on the terraces and players, who were nearly all British. There was no such thing as a Champions league player. No agents. But for all that, a good player is a good player. Harry Kane would've been okay, not sure about Dele Alli.
Thankfully things have improved! The program Alan shearer did on concussions in football was an eye opener and it was no secret that the stitches on the ball cut the players heads open constantly.