1) I would be happy to accept your invitation. I was always looking forward to participate in the exhibition.
2) I will response your invitation on conference. Can you submit your materials, consideration and comment? Your earliest response would be appreciated.
3) I very much regret I can't accept the invitation. Attend to talk, since next month it comes the middle of the academic semester and i have lectures to present.
4) I regret I cannot accept your kind invitation. I hope i'll be able to visit you some times in the future.