My suggestion is, do not add Wizard or Cleric levels "on top of it".
Instead, replace some of its HD with Wiz/Cle levels, or just apply Wiz/Cle spellcasting levels over the Hag stats.
I try to explain better...
First, notice that the Green Hag in both 3.0 and 3.5 has 9d8 HD and CR 5.
If you look at it in terms of "I'm going to add Wiz/Cle levels on top of it, starting from class level 1, then 2 etc...", it looks like you are adding good and useful powers (the spells) and also additional HD (i.e., hit points) therefore you should increase the CR.
But if you look at it from the point of view of an NPC Wizard (or Cleric) of already level 5 (which would have CR 5), adding Green Hag "on top of it" benefits mostly in terms of those extra ~50hp. The special abilities may not even be worth a CR increase, because the spell-like abilities (except Invisibility) aren't useful in combat. The Strength drain is definitely more useful tho! (Note that this considerations are for an NPC, but if your plan is to use this as a PC then those spell-like-abilities are indeed worth a LA)
Therefore what I would do, is:
- first of all decide what is my target CR. I assume yours is minimum 5, since this is the CR of a normal Green Hag.
- determine the known/prepared spells and daily slots of a Wizard or Cleric NPC of level = target CR
- use the stat block of the Green Hag as-is, including all the special abilities, but except the hit points
- calculate hit points as the Green Hag's original value + roll as many hit dice (d4 for Wiz, d8 for Cle) as needed to reach your target CR
- (optionally) increase all skills by CR-5, and give it an extra feat if CR = 6, and one more feat for every 3 levels thereafter
For example, if you decide you want it CR 7, you'd basically be looking at the Wiz or Cle spellcasting table, write down how many daily slots at 7th level, and fill them with prepared spells*. Then add 2d4 if Wiz or 2d8 if Cle to the normal hit points of a Green Hag, and use the rest of the Green Hag as-is**. For more precision, add 2 to all skills, and pick one extra feat.
*for Wizard you don't actually need to determine all known spells, just fill those slots with spells of appropriate level, anyway the PCs will never know what spells the Hag knows but hasn't prepared
**another option would be to replace ALL hit points with those of the class, i.e. 7d4 for Wizard and 7d8 for Cleric, but notice that the latter is the same as adding 2d8 on top of the Green Hag's hit points
