Its financial underpinnings and menu both beefed up, Del Taco Inc. is launching an expansion drive that will combine internal growth with an aggressive acquisition plan. To make it work, Chairman Kevin K. Moriarty said he has hired longtime industry executive C. Ronald Petty to take over daily operations of the Laguna Hills chain, a move that frees Moriarty to pursue takeover targets. Petty, 54, who worked with Moriarty at Burger King Corp., will be Del Taco's first president in almost five years. Moriarty had handled both daily and long-range management chores since pulling Del Taco out of bankruptcy in 1993. Industry analysts say that the two men--who helped Burger King grow from 1,700 outlets to more than 4,000--are a formidable team. Moriarty said he wants privately owned Del Taco to double in size over the next two years.