Mumble years ago, I did a conversion of a lot of 1e material to Champions (pre-Hero System). One of the things I learned is that you can try to do as exact a match as possible from the source, but it's actually a bad idea. You end up with spells in some kind of framework, with all sorts of advantages and limitations layered on to try to exactly match the original. It gets ridiculous. You're better off using guidelines like those cited about from Killer Shrike for general power levels. Then do a build of what feels right at that power level. Once I saw what has been done with Hero System Grimoire, with various options for every spell, I decided that I truly didn't need one definitive version of each spell. Having reached that conclusion, I knew that I could go for the right feel rather than exact mechanics.
With that said, there are some things you want to do. You want to scale damage, range, area of effect appropriately. That doesn't mean matching the source. That means that if one spell has twice the range of another in the source, it should probably have twice the range when you convert it. Similar concerns apply to other features of a spell.
Monsters are harder to convert. The big reason is what DShomshak commented on above. Simply, the Hero System has mechanisms for modelling things that don't all appear in most games. If monsters are lacking in defenses against certain types of attacks, those become weaknesses. If every monster converted from a particular source is lacking those defenses, you've increased the value of those attacks because they become disproportionately effective. Monsters really need to be a build from scratch to match the intent of the original.