There is no provision in the standard rules for making magic items that give non-enhancement bonuses to stats. I would strongly advise against allowing them and here is the reason why:
Belt of Epic Strength: +12 Enhancement bonus to strength, 1,440,000gp
Belt of Not Epic Strength: +6 Morale Bonus to Strength, + 6 Enhancement bonus to strength. (6^2 x 2,000 x 2 + 6^2 x 2,000 x 1.5) 126,000gp
They do the same thing, but one is less than a 10th the cost of the other.
Belt of Epic Strength: +12 Enhancement bonus to strength, 1,440,000gp
Belt of Not Epic Strength: +6 Morale Bonus to Strength, + 6 Enhancement bonus to strength. (6^2 x 2,000 x 2 + 6^2 x 2,000 x 1.5) 126,000gp
They do the same thing, but one is less than a 10th the cost of the other.