A Canadian investment company has bought 5.75 percent of the stock of Avemco Corp. and has notified the Bethesda aircraft insurance and financing company it plans to buy more.

Avemco Chairman Arnold H. Johnson said the stock was purchased by Unicorp Financial Corp., a Toronto company whose president George S. Mann is its controlling shareholder.

Beginning in June, when Avemco stock was selling for about $5 1/2 per share and continuing into this month at prices up to $8, Unicorp bought 142,800 shares of Avemco.

In a report filed with the Securities and Exchange Commission, Unicorp said it intends to acquire additional shares to bring its holdings up to at least 10 percent of Avemco's outstanding stock.

The Canadian investors told the SEC they have "no present intention" of seeking a voice in Avemco's management or a seat on its board of directors.

Johnson said he learned of the purchases this Tuesday when Mann called on him. Unicorp's report to the SEC -- required when anyone acquires more than 5 percent of a publicly owned company -- was filed Thursday.

Unicorp's Canadian operations include a trust company, a bank, a management company, commercial real estate and securities, the SEC report discloses.

Avemco is one of the nation's largest insurors of private planes, finances the purchase of small aircraft and markets related products. Johnson said he believes Unicorp bought the stock because Avemco shares were selling below book value and the company's earnings in the first six months of 1979 were up 40 percent.