Get a short aluminum billet intake to improve carburetion and motor performance- under $20 shipped
You may also want a newer 66 speed carb- also about $20 shipped- look for shutoff valve on the carb and the word "Speed" or "66 carb" in the ad- it's got a bigger bowl, a bigger port area and a larger jet to let in more gasoline. But the billet intake really makes a bit more difference if you have to choose
Narrow tires will give you a better roll, a lighter bike and less vibration- you can get 26" all the way down to 1.25 inches these days, but those are rare and more expensive. An older !.75 style tire is out there that doesn't cost much and has a nice road tread for a decent roll, or more expensive and generally smoother 1.50 tires are more polular with all cyclists these days . If your rims are alloy and not steel- the general rule is probably look for tires with the size described in decimals like 1.75 and not fractions like "1 3/4"
I've got narrow 27" and 700c tires on my two cruiser builds, generally about 1 1/4" wide- like an older ten speed road bike, while I use a modern lightweight cruiser frame for motor, tank and chainstay clearances