You will have to buy a new SCSI card... and an adapter cable.
Fortunately, non-RAID SCSI has gone out of style, so there are plenty of old cards availible, here is one that should work very nicely for you:
http://store.yahoo.com/livewarehouse/adahpcschoad.html
*Note: I have no experience with this store, they are just someone random I pulled out of google. But the price seems really nice.*
The modern scsi controllers can talk to old scsi devices, so unless you get an LVD controller, you should be great (the link I gave you was not for a LVD controller). So just find an adapter, or a special cable that will convert from 50-pin SCSI-2 to 25-pin SCSI-1