I would make a slip, made from the same plaster as the model was made from, and use the slip to "glue" my two pieces together.
Scoring the inner surface slightly giving the slip a bit of a roughened surface to bind to.
When the two halves are brought together you want to see the slip forced out of the joint. Just wipe off the excess with a damp cloth.
If you don't see the slip being squeezed from the joint then you didn't use enough.
By using the same material to bind the pieces together as what the sculpture was made from you won't have to worry as much about discolouration.
Hard part will be holding the two pieces together while the slip dries.
Don't use masking tape! Masking tape will stain your material.
If you do go the tape route, use a good quality painters tape as you can leave it for hours, or days, and not worry about it leaving any residue behind.
Here's a quick sketch of the steps I would take.
Find a narrow headed screw (such as a flooring screw) and measure its length. We will call that length "d".
Drill a pockets slightly larger than the head of the screw into both halves of the plaster cast to a depth of 1/2 d.
Fill both drilled pockets with your slip compound.
Insert the screw into the pocket such that half the length of the screw is sticking out the top of the cast. The part that is sticking out will what will hold the other part of your cast.
Align and bring together both halves of the broken cast. And squeeze them together. You want to see the slip squeezing out of the joint. If you don't them you did not use enough.
Once the pieces are together you can use a damp cloth to wipe the excess slip from the surface.
Bind the two halves and allow to dry.
I'm not sure how big of a piece we are talking about in your case. If it's a lot smaller then instead of the screw you could use a piece of aluminum wire to act as the screw. Remember to form the wire into a loop at both of its ends. Once the slip dries the loops will be anchored into the dried plaster and keep the two pieces from pulling apart.