I tore up the front shock on my bicycle and broke the lower caseing. can i rebuild it with a different bottom?

Depends on the fork. If the bike originally cost less than $600 then it would be MUCH cheaper to buy a new fork... or a new bike.

Lower end bikes and forks are intended for occasional riding on the road and offroad on groomed surfaces. Anything more than that and you can count on something failing sooner than later and costing more than the bike did to repair it.

It's cheaper to replace the whole thing.

http://www.amazon.com/s/ref=sr_nr_p_n_feature_keyw...