GS500e and GSF400(bandit series) are vastly different bikes from bottom to up also GSF was way more powerful bike, I would humbly advise, NO GS500e engine cannot fit in a Bandit chasis.
Now comming to your Question # Yes GSF engines are readily available in used spare parts market (Bilal Ganj) in lahore, but its a pitty that you will have to spare some time for you self to come here and buy one which is in good condition ...