Though, not direct sequels to '64'...New Excitebike. Excitebike 64 was so good.
The developers of Excitebike 64 made a spiritual sequel of sorts, in the form of 'Nitrobike' for the Wii. Though the game's decent and fun; but not that good (visuals were "meh", problems with slowdown sometimes with end affecting the whole tricks/scoring..but, still plays similar to EB64 and has online-multi).
(trailer)
There's also the WiiWare game 'ExciteBike: World Rally' from Monster Games (ExciteTruck and ExciteBots), which is like the original NES game but eith updated visuals, online multiplayer and a level editor:
(trailer)
EDIT:
Posting this, made me go out and rent (again) NitroBike...and it wasn't as "average" as I remember. Basically like ExciteTruck/Bots, but without detours. Visuals (especially character models) are really generic, there wasn't slowdown in the races i tried neither. Online is..well, is there..but noone playing.
So if you liked '64' you might enjoy this one too.