One big difference--you will join the "Wrong size belt Club" and gain a whole lot of trouble trying to find a belt that will work correctly! Maybe a stock new belt will work. Best to stay with the correct sizes, than to open up a whole "new can of worms"! BTDT: Rusty J.

Might be better off installing a Honeywell HTF7000 engine to power your deck.

Depending on the pulley ratio's, you would probably spin the blades to the point of severe danger.
Blade tip speed is regulated so you don't have a potential "bomb" going off if you strike an object.

Probably better off getting a new belt that doesn't slip.

Ah yes. It will shoot the walnuts even further. Sounds like a reason to do it as well as a reason not to lol.

Thanks for the input. The belt is new to this year so my plan is to get back into the deck and see what looks ths most worn and maybe replace it. 23 year old machine and perhaps the age is starting to show.

***"23 year old machine and perhaps the age is starting to show."***
Yes, perhaps just a new drive pulley of the OEM size would help.
Years of wear can reduce the "apparent diameter" of the pulley, resulting in a slightly reduced blade speed.
Of course, wear to the driven spindle pulleys would reduce their apparent diameter also and maybe cancel out the effects overall.

Smaller pulleys all the way around would result in the same blade speed, if they wore equally.

BUT it would also result in a shorter run by the belt. Half an inch over 80 or so inches might just make a difference...

Yes, that is what I was referring to......that any reduction of belt speed due to wear in the DRIVE pulley.....might be cancelled by wear to the DRIVEN pulleys, resulting in the same net belt speed.......but with some slack appearing in the belt train.
Replacing just the DRIVE pulley might put your belt (and blade) speed up near where you want it. But without a tachometer (or some very careful measurements along with accurate calculations) it really amounts to theory and guesses.

This might be a "reach" but somewhere under there you'll find a spring or springs which maintain belt tension. Springs can get tired. Investigate replacing spring, may not be too much $$.

Also, you mentioned that you have a "new" belt, OEM or "aftermarket, yeah it fits?" :-)

Good luck,

Ev