This phrase is a typical example of what I call Italianish, English words Italians seem to love using, but also words that lose their meaning once translated in English as such. Sorry it's one of my pet peeves about the latest "language" trends.

The meaning is
the fair is a magnet for "qualified leads" (in other words potential buyers)

the XXXX fair is a qualified leads magnet that attracts 10 million visitors and potential buyers each year.

If you must keep the "engine" you may wish to consider

1 - the fair is a lead generation engine that 10 million visitors and potential buyers each year.
2 - the fair is a sales generation engine that attracts 10 million qualified leads each year.

In sales/marketing term, a qualified lead is a customer proven to have an interest in the sector or the financial means to purchase the product, depending on what the item is.
i.e. in the cars business, a qualified lead is someone who has a certain credit score; in business equipment sales, a qualified lead is a company that either works with that type of equipment or resells it to consumers.

http://www.businessdictionary.com/definition/lead-generation...
lead generation Hide links within definitionsShow links within definitions
Definition
Process of collecting names and contact information about qualified prospects which will be contacted by the salespeople for generating orders. It usually involves direct response advertising and telemarketing.

http://www.answers.com/topic/qualified-lead
Prospective customer who has a demonstrated interest in the product or service being sold, the ability to pay, or the authority to make a purchase decision. For example, someone who has returned a reply card requesting a salesperson's call is a qualified lead, in contrast to someone who is contacted by a salesperson without any prior verification that he/she can pay, can make the decision to buy, or has any interest.