When I had a MillerMatic 200, I used to use .045 liners on .030 wire, as they would not clog up. They worked fine. They were Tweco guns.
Now I have a Lincoln SP175+ and wonder with this light little cheesy gun if one can buy only the liner or if you have to buy the whole gun. The welder is acting a little cranky, and I'd like to just put a larger liner in there.
The manual shows only 3 liners available:
M16291-6 .035 liner M16291-1 .045 liner M18971-1 .035 liner for aluminum or stainless (teflon?)
So it seems as though you could certainly put in the .045 liner, yes.