A MKII barrel will fit, but a used MKII will run you about the same cost as a MKII barrel and a gunsmith fitting it.
I would consider trying a different magazine, or retiring the pistol in favor of a used MKII. Depending on the wear/damage, it may be able to be fixed, but I have never heard of a feed ramp on a Ruger .22 auto pistol wearing out. Without actually inspecting the ramp myself, I would wager a spring change in the bolt, or a magazine change will revitalize the pistol, especially if you can't pinpoint damage.
Numrich used to have MKI barrels for $45 or so, but they are long sold out. They also used to have barreled recievers for MKIIs for $150 or so, also sold out.