Bill Britt, who calls himself the "Happy Hermit of Chestnut Hill," is looking forward to being a little bit happier, even though he may be evicted from the wigwam he has been living in for 18 years near a reservoir in the well-off section of Boston. He says he will let Mickey Rooney portray him in a TV movie for $29 a week for life. "I can live anywhere on $29 a week as long as I have outdoor living," the hermit said. If he loses his legal battle over his eviction, which he plans to wage more vigorously with Rooney's promise of help, he dreams of moving to a shack on the coast of Massachusetts, perhaps on Martha's Vineyard. He would also like a motor scooter with a sidecar for the "big collie dog" he would like for companionship. Rooney, who is in Boston starring in a stage role, has not commented on specific terms of the agreement, but said his production company will put together a two-hour special on Britt's life. "This is something the world should see, realize and know," Rooney said. "There are so many avenues of pain and so many avenues of hope . . . (in Britt's story) that I believe it will probably be . . . one of the most powerful pieces in which I will ever have the privilege to appear." For his part, Britt, who hopes his eviction battle will call attention to the homeless and hungry, said: "I'm thrilled. I think he will be excellent in the role."