"which missing item would complete this alpha decay reaction?
"

This is alpha decay not beta decay. 

Fermium 257 decays by ALPHA decay into 253 Cf which has an atomic number of 98 

So your reaction is 

257Fm --> 253Cf...+ 4He 
100................98.........2 

Interestingly Californium does undergo beta decay. 

253Cf ---> 253Es + e- 
98...............99 

Hope this helps.

Lx bxh