Ok , did a bit more reading and this is what i got...
piaggio tyre is old inch based design... 3.50-18, in old inch based designs , tyre width and height are same, so Piaggio has the tyre which is 3.50 inch wide and 3.50 inch high.
While delux 90/90-18 tyre is 3.54 inch wide and 3.18 inch high as i mentioned above.
So both are about same width, but delux tyre is a bit low on sidewall height. Now this might make you wonder what to choose if you want a new metric based tyre and don't want to loose height also.
First option is straight forward , get a 100/90-18 tyre. It will be 3.93 inch wide and 3.54 inch high.
If its too wide , and you want slimmer yet higher sidewall tyre then you can try to find a 90/100-18 tyre which should be same as piaggio tyre , aka 3.54 inch wide and 3.54 inch high , but i am not sure this is easily available or not.