I'm not 100% certain I'm remembering this right, but I'm pretty sure the Massey rims that used the same 8-on-8 bolt circle also had a 6" pilot hole - identical to Allis. The Fords that had the same bolt pattern are more common, but the Ford pilot hole was 5.5", so if you find/use Ford rims you need to carve out the pilot hole a little.

Are the 28's on it now 18.4's? You'd probably be able to find 28" rims without too much trouble. But finding 28" rims wide enough to accept 18.4's would be tougher. Probably easier/cheaper to fix or replace the power adjusts. Because I'm cheap/lazy I don't bother trying to find wheels/centres with matching bolt patterns. I buy whatever used tires/rims/centres are cheap and have decent rubber and solid rims. Then get discs cut on the plasma table at work with the right bolt pattern and centre hole. Cut a disc out of the old centre, weld the new one in, a little grinding/paint, and they look pretty good. I'm sure the purists here would criticize me, but it works for me. Cheaper and easier than trying to find the right centres. Pic below is a set of 18.4-30 rims and tires from a log skidder that I converted for the 180. I paid $200 total for the rims and tires. Discs to convert were less than $100, so in total was a really cheap way to get decent rubber and rims.

How 'good' are his tires in actuality? If (as it sounds) he's not worried about keeping things stock, I'd personally be inclined to go with 18.4-30's. You don't gain any appreciable performance/flotation by going to 30's (there's only something like a 3% difference in contact patch area), but you're at least converting to a much more common and easily-found tire size. Loads of good, used 18.4-30's are always available, whereas 28's pretty much have to be bought new every time. And the 180's/185's look (in my opinion) way better with 30's on them. That extra inch or two at the rear makes them sit more level. The 28's Allis put on looked a little squat in the back - especially once worn (again - just my opinion).