OFF East Seventh Street in the East Village, through a doorway marked with a faded Hebrew Actors' Union plaque and up a flight of stairs, a slim man in a dapper suit kept dashing madly into the long, empty room, rehearsing slapstick stage entrances. The man mugged like Jolson and mimed like Chaplin. He ladled out Borscht Belt bits and vamped vaudevillian. All this, on Wednesday afternoon, as sepia faces from the past stared at him from photographs on the walls. If Maurice Schwartz, Aaron Lebedeff, Reizl Bozyk and these other legends of New York City's bygone Yiddish stage could speak, they may well have said: Not bad for a nice Episcopalian boy. [ABSTRACT FROM PUBLISHER]