When I worked in naval architect offices, we usually ran calculations for masts based on empirical formulas that have been around for a long time. Stability played a role but if I remember the formula correctly there was a simple surrogate formula that added increased load factor to the formula that compensated for higher stability. While today there are sophisticated computer programs to size spars and rigging, They really would not be necessary if you are simply replacing the spar on your 1960's era Seabreeze.
If I remember correctly Skene's had a pretty simple chapter on spar design.
As to making a decision between different spars, assuming that you are not changing your shroud attachment points or your spreader positions, the key numbers that you will want to pay attention to are the axial area of the spar, and its moment of inertia. Masts, especially in masthead rigs like yours, take enormous compressive loads and the axial area is the determinant of the axial load that the spar can take if in column. They also are subject to bending forces, and the moment of inertia refers to the stiffness of the spar, and so its likelihood of remaining in column.
For the same rig geometry, smaller dimensional spars are not usually lighter than bigger sectional spars since it takes more wall thickness to achieve the same necessary stiffness. Weight savings can only be acheived with smaller dimensional spars when the rig geometry is adjusted to reduce panel length so that the required stiffness can be reduced and perhaps spar tapering can be employed.
If you want to wipe out your beer supply and wear out your thinking chair you can also use the formulas from German Llyods:
Whatever you do, you are probably looking for a pretty conservative design. If worse came to worse, you should be able to determine the area and I (moment of inertia) of you old spar by looking up its dimensions and wall thickness at Dwyer or Kenyon, since most of those old spar shapes are still in production and use those numbers to make a judgement call on which spar section best suits your goals. (BTW within the constraints of conservatism likely to be employed in designing a spar for a cruising boat, fatigue should not be an issue since the safety fators are much higher than the sophisticated formulas used with high performance oriented computer aided design programs.)