replacing power supply capacitors

I am rebuilding a BGW 750A and need to replace some caps including the power supply capacitors. I am not looking to rebuild this to original specs and if a change would improve performance I would like to do that. I am having a hard time finding an exact replacement and wonder if I can increase or decrease capacitance and to what degree is acceptable. Currently there are two GE 21000uF 85V large can screw type capacitors. Would I be better to move up to 22000uF or down to 19000uF and what difference will it make? I have checked some of the online large electronic supply companies and have been unsuccessful at finding an exact replacement. Would I be better going to more quantity of caps? Thanks
 
Thanks for the quick replies. Apex Jr, I measured mine and they are 3" diameter x 4 1/2" tall. I need 6 as I am refurbishing 3 amps. I really would like to find a direct size replacement, but if am unsuccessful, that seems like a good price. I think I have enough forgiveness in the wiring to accommodate those. Do you carry the clamps also?

Thanks
Devin