Maybe a better question would be, is my Digicel SIM card (phone number) still valid?
If it has been several years since you last used it BTL may very well have cancelled the number(SIM). Email them your phone number and see if it can activated still.
Personally I think I paid about $30 for a very basic ublocked GSM Quadband Nokia 1661 about two years ago and then about $50Bzd for the SIM. I've kept the SIM active by periodically topping it off with $5 Bzd about every 6 months or so.
You may want to consider renting a phone if you dont care about owning one. It seems to me a number of people here on the board offered links to very reasonable rental companies on Ambergris.
What model Nokia phone is it? I know you mentioned you could find a replacement but I've had excellent luck finding them on Amazon.
Here's a link to Digicell if you dont already have one handyhttp://www.digicell.bz/your-account.html
Note Edit: the 1661 is no longer a current model but you can buy it on Amazon still for about 35 to 45$. The simialr current model is the C1-01.