Investors to Buy 28% of Medical Firm’s Stock : Deal: If Siegler, Collery also exercises an option to purchase preferred stock, it would be able to appoint two directors to American Health’s board.

A New York-based investment group has agreed to pay $6 million to acquire 28% of the common stock of American Health Services Corp. as well as options to buy 85% of the firm’s preferred stock.

American Health, headquartered in Newport Beach, operates medical diagnostic imaging centers.

According to a document filed Monday with the Securities and Exchange Commission, all the stock would be purchased from Harpener AG, a German firm that until recently was controlled by OmniCorp., a Swiss conglomerate that has filed for bankruptcy.

Siegler, Collery & Co., a New York investment firm, has agreed to acquire Harpener’s 3.6 million common shares of American Health for $2.8 million. It also has obtained an option to buy 3.2 million preferred shares that are convertible into 3.2 million common shares, plus warrants to buy another 423,400 shares from Harpener for $3.2 million, according to the SEC filing.

“We are doing due diligence now to make sure we want to make the investment,” said Gary N. Siegler, the principal owner and president of Siegler, Collery.

Siegler said his company, acting as a general partner with other investors, has promised to decide by Friday whether to proceed with the preferred stock purchase. The deal is expected to be completed by April 26.

If the Siegler group purchases the preferred stock, it would have the right to appoint two directors to American Health’s board. Last Wednesday, four board members resigned, including three who were affiliated with OmniCorp. Another board position is vacant. American Health’s only remaining directors are E. Larry Atkins, the company’s chief executive, and Philip D. Green, a Washington attorney.

In addition, Atkins said Siegler would be entitled to elect a majority of American Health’s directors until the company is no longer in default of an $800,000 payment it owes to its primary creditor, Philips Credit Corp. of New York.

Atkins said Monday that the company had been pleasantly surprised by Siegler’s action. “Overall, this change could be good for American Health. It is rather exciting,” he said.

Atkins noted that the Siegler investment group has experience in the diagnostic imaging industry, since it also owns a controlling interest in Medical Resources, a New Jersey company that is similar to American Health.

Siegler said his firm has also invested in Hexcel Corp., a Northern California manufacturer of structural materials for the aerospace industry, and National RV Inc., a recreational vehicle manufacturer in Riverside. Atkins said the Siegler group has “indicated that they have additional funds to invest in American Health.” By contrast, he said, OmniCorp’s financial troubles prevented Harpener from giving American Health a helping hand since it acquired a big stake in the company in February, 1990.

Atkins said in a telephone interview that he was in New York Monday to raise another $1.5 million that American Health needs to complete the purchase of a Swedish medical device known as a Gamma Knife. He said American Health already has invested $2.5 million in the machine, which uses a laser-like beam of radiation to dissolve brain tumors.

American Health has said the cost of buying the Gamma Knife is mostly responsible for the company’s cash crunch. Atkins said the company ultimately hopes to generate considerable revenue by operating the machine in a treatment center at the University of Maryland Medical School.

For the nine months ended Sept. 30, American Health reported revenues of $24.2 million and a loss of $2.3 million.