Leece-Neville is nice enough to make an external regulation conversion kit for this series of alternator. It should be available from just about any Leece-Neville dealer.
I ordered this conversion kit from my local alternator shop who is a Leece-Neville/Prestolite dealer. The total price for this 90 amp marine rated alternator with the conversion kit was about $325.00. About half the price of the Balmar Series 81 (which is now discontinued).
Please be careful when buying on-line as there are numerous Chinese made clones of these alternators. They are not nearly as well built nor as robust.