To measure the blade's pulley "pitch diameter": make a chalk mark on
the pulley and belt just where the belt "exits" the pulley, rotate the
pulley one revolution so the belt's chalk mark moves away from the
pulley, and measure the distance on the belt from its chalk mark to the
pulley's chalk mark. This is the pitch circumference, divide by pi for
the pitch diameter.

Buy a new pulley whose pitch diameter is twice this. Pulleys *are* sold
by pitch diameter. Maybe not the ones on the rack at Ace Hardware, but
the ones at MSC, McM-C, etc.

HTH,
Bob