Typically a press fit of that type would be in the 0.003" range.

In manufacturing,  Interference fits rely on a complicated equations which takes into consideration many factors. A few are, amount of torque that is applied to the joint, depth of fit, material dissimilarity, etc. There are more criteria, all engineering jargon
The engineering formula is too complicated to discuss here besides I hardly ever used it at my job.
The simple guide lines I always used which rarely got me in trouble was for diameters 1” or less are; .001”- .002” with .0015” being the median.
As the shaft diameters increase the hole size gets smaller, eg 4” shaft .004” .
If you want to be safe use a shrink fit, go on the high side maybe .002” - .0025” interference and heat the pulley, cool the shaft.
As for aluminum pulley’s go for the tighter fit with a shrink fit.
I would use a micrometer to check the shaft size on the pump and a telescopic hole gauge on the pulley that fell off to see what the issue was in the first place. Could have  been  a QC problem, shaft size minimum, bore size maximum.

Pump seizing or not the belt should be screaming and jump off before the pulley spins.

Did a little further investigation on this today.

The pulley hub has a 1” depth.
I used a sharpie and colored the shaft of the pump.
I was able to push the pulley onto the shaft over 1/2” in its normally mounted direction.
I then flipped the pulley 180 degrees front to back, I could only get the pulley to line up on the taper.

Pulley hub is definitely shot on rear half.
Found a replacement from TimC today.

Looking at the hub how it is attached, looks like its pressed in and staked.
Pulley appears to still be true, just hub is damaged.
Can the hub be removed and replaced?

Now if I had Rudi’s skills I’d fix it in my sleep.😁

Dan , if you have access to a lathe there is no problem pressing in a new hub. The new hub should be at least welded in place and the pulley chucked in a lathe,  TIR verified, the the hub bore machined to the press fit diameter.
OR . . .  bore the existing hub out to the next nominal size press in a bushing and bore it out in a lathe, many ways to skin a cat.
That’s the best way to ensure everything is concentric.
It all boils down to the time line to fix it or money to get a new one.
If you lived closer and not in another country I’d fix you right up for ya.

I would make the new bushing an interference fit and to be safe should be welded , it stays with the pulley when removed.

This past week I installed the pulley on my PS pump.  In the past when I installed these pulleys I used some oil on the shaft and always had a tough time with the pulley turning during installation.  After reviewing a YouTube video that recommended using a EP grease on the shaft, tired t and found the pulley went on smoothly without any difficulties. 
Note that the old and new pump shafts had the same diameter.