You need to know the HP requirements for the sander you have when it is heaped with wet sand,which is the toughest to carry and spread.You might have to gear the electric motor down so the conveyer and spinner spin slower than before to meet the requirements on HP.I would think it will would work,as long as you get a pulley made to convert the chain to belt drive and run some thick cables to the truck battery,with a HD solenoid.Is all this worth it,and will you have saved any money over just buying a new briggs 8.5HP for around 400 and slapping it on,you'll have years or trouble free service.
I think it could be done, but like John said, the conveyor chain gear box, needs to be geared down. Since changing the gear box would be cost prohibative, going to a different sprocket and chain combo should do the trick. It may take some T&E but I think it would work fine. Is it worth it, the smith motor is about $ 250.00 plus cables and solenoid, and connectors. In the end I think it will cost as much as the new briggs, so you will have to determine if thats what you really want.