I think the intake hose size on the 1250 & 1260 pump is the same, 1 inch or 25/34mm.
The output hose size is different, the 1250 is 1/2" or 12/16mm and the 1260 is 5/8" or 16/22mm.
You should only have to get new output hose & the spraybar if you use it.
There is a possibility that the Elbow Connection fittings (Letter I), located on the canister lid, might not 'fit' the 1260 pump. I only say this because the part numbers are different in the owners manual. If that is the case, you will just need to purchase those parts. They come shrink wrapped as one part, #7675950.
Hope this helps.