That is a Yamaha BYZ301, Yamaha made a BYZ201 (mini), 301 (yours), 401 (mine) and 501 (top of the range with all Shimano parts).
ultrasham241 also has a 401.
Absolutely worth restoring, I had a BYZ401 one as my 2nd ever BMX and still have one restored to original with the original parts on it.
This is it....
