Putting a new leadpipe on would be the cheaper of the two exchanges.
There are two modern leadpipes, which will fit: those for the 981 and for 982.
The one which would be relevant, is the one for the 981. It starts and ends with the same dimensions as the other one, but it expands much faster. Thereby allowing for a bigger and warmer sound.
The effect of installing the 981 leadpipe may potentially satisfy you so much, that you will not want to spend the money on the bell exchange.
Klaus