From The Turbine Pilot's Flight Manual:

"V1: Takeoff Decision Speed

Let's start with V1, takeoff decision speed. Simply put, it is the "go or no-go" speed. If an abnormality occurs before V1 is reached, takeoff is to be immediately aborted. If an engine failure or other abnormality occurs after V1 is attained, takeoff is continued and any problem is treated in flight. Most airline and corporate flight departments have historically used V1 speeds based upon "balanced field length" for their particular aircraft. If an engine failure occurs exactly at V1, the distance required to abort the takeoff and stop is the same as the distance required to continue the takeoff. In familiar multiengine terms, this means that accelerate-go distance equals accelerate-stop distance."

In your example, you got the engine fire indication right at V1. Therefore, it would not only be legal, but also prudent to abort the takeoff.

Furthermore, in my first Captain PC, the examiner started a discussion by asking whether or not there were any circumstances in which I would consider aborting a takeoff after V1. After quite a bit of discussion, it was decided that under specific conditions, you might in fact use your best judgment and decide to abort the takeoff. It's the reason that airplanes aren't flown solely by computers, and that is what Captain's Emergency Authority is for.

In a Saab 340, with at least 10,000ft of runway left, with a possible engine fire, and no other extenuating circumstances - I'd venture to say that I absolutely would abort the takeoff. If it were a B747, with only 7,000ft of runway left - that would be a different story.