Yeah, leave it as it is.

And, the difference I've seen between CT90's & CT110's in the U.S. is how well they are tuned. There are a couple of CT90's that can "clean my clock" when I'm on my 1983, CT110 (and I think I've got my CT110 tuned well), and we're about the same in rider dimensions (weight & sizewise).

Find a beater bike with a bad engine and do the 140cc Lifan convesion.
Maybe even paint it some neat color.
Don't tell the wife that you can go faster, it'll just look a little different.
That will allow you to keep up with traffic on two lane roads and avoid the 'moped' feeling on the back of your neck.

And, as much as I like my dual range sub transmission, I have several friends that own 1980, CT110's (no low range, like your posties) and have just learned to ride faster in first gear going up steep hills. Hey, if you're still young enough, you'll live forever, right?

Paved roads, just more needless government spending...

I pick up my mail in Santa Barbara, California, USA.