I am not familiar with the Wards Riverside, but I can tell you a few things that may relate to this.
I have 5 Puch motors all with a 38 mm bore. But, the bore size varies on 3 of them. Some are marked with a 3 on the top of the cylinder and one marked with a 33 and one marked with a 1. The pistons have similar markings. They vary in diameter a small amount, on the order of less than a thousandth of an inch.
The piston to cylinder clearance for a chrome bore Puch 50 cc motor is supposed to be from a minimum of .0008 inch to a max of .0016 inch. The cast iron bores are supposed to fit looser (from .0014 to .002). But, there are I believe 5 different sizes of 38 mm chrome bore cylinders, each with a number on top that specifies which of the five it is. These cylinders vary enough that my piston marked 3 will go into the cylinder marked 1 but with NO clearance. The number 5 piston (and cylinder) is .0016 inch larger than the number 1.
If I were to buy a piston for my 38 mm bore Puch, it could come in 5 different sizes. Unfortunately I cannot bore out a chrome cylinder at all. You can bore a cast iron cylinder, but a hone is not a good way unless it is a "rigid hone". A non-rigid hone can very easily make your cylinder less than straight. It may get larger on the top or bottom, but even more likely, larger in the middle. A rigid hone used properly will make it straight. The piston is supposed to be larger on the bottom (skirt)
Your best bet would be to find out what the piston clearance is supposed to be and then take the piston and cylinder to a qualified shop and have it bored properly. Alternatively you may find that there are various "sizes" such as mentioned above and then order the correct one, if the Riverside uses a sizing scheme similar to the Puch stuff mentioned above. Keep in mind that you positively do NOT want to bore an aluminum cylinder with a chrome bore.