A person with skills can build a new foil but given the effort required you should avoid the plywood or steel core that others have suggested. Plywood isn't strong enough and is subject to fatigue. If steel wasand appropriate material you wouldn't be replacing the board. Read the article in building a new board by J. R. Watson www.mothboat.com/CMBA/Building/foils.htm
If the construction techniques he describes produce an underweight board you can use a hole saw to insert a couple of lead hockey pucks near the tip before you glass the board. Each 4" diameter plug would weigh close to 6lbs.