An economic way to improve the shifting of that Rotor HG cassette would be swapping it for a Sunrace one. Your son would still have a decent 11-50 ratio and those aren't that hard to find online.
Also, the Superstar Slackerizer -2° headset is a wonderful and inexpensive way to modernize a bike's geometry. Shipping charges might be a problem though, as Superstar seems to dispatch their products overseas via Olympic swimmers judging by their pricing scheme.
July 2, 2021, 8:06 p.m. - Luix
An economic way to improve the shifting of that Rotor HG cassette would be swapping it for a Sunrace one. Your son would still have a decent 11-50 ratio and those aren't that hard to find online. Also, the Superstar Slackerizer -2° headset is a wonderful and inexpensive way to modernize a bike's geometry. Shipping charges might be a problem though, as Superstar seems to dispatch their products overseas via Olympic swimmers judging by their pricing scheme.