substitute for msg in a recipe

Like others have said, you can leave it out.
Having said that, I have lately been adding Umami flavors through various means, such as a sprinkle of nutritional yeast in cooking to add a little depth of flavor & nutrition, especially when I am cooking vegetarian. There are different opinions about naturally-occurring "bound" glutamates (nutritional yeast) being ok vs synthesized (msg); you'll know best what you can tolerate. (Here is an online discussion related to my google search for "glutamates and nutritional yeast": http://www.msgmyth.com/discus/messages/3/1044.html?1344650804). I'm sure others will weigh in. Essentially msg ("Accent") enhances perception of flavor (I remember my mother using it until all the news reports came out). Lots of foods have naturally-occurring glutamates, so if you are required to eliminate them entirely then no, there's no substitute, but it's not essential anyway.

The only thing I notice about NOT using MSG is I won't have a headache after I eat. Really no need to use it.

If you can't handle MSG, don't replace it with anything. It's a naturally occurring substance.

I don't know why people are suggesting the replacement of MSG with dulse or kelp.

MSG is a synthesized version of the same component that is found in kelp. MSG was originally extracted from kelp by the Japanese over a hundred years ago and eventually was replaced by the synthesized production which is chemically identical.

It was not a "cheap way to enhance flavors because spices were expensive" [sic].

MSG is a cheap way to enhance flavors because spices were expensive, but I found that either skipping it (esp if you already used to no msg, you'll not miss it), and/or just adding a bit of more flavoring and spices from what is already in the recipe should work. Adding other natural source of msg flavor (mushroom, kelp, etc) might add new uninvited flavor.

Powdered dulled (a sea vegetable) can replace msg.
Be careful...as the original is from glutamate, found in seaweed, this season veg may also disagree with you.
Use 1-2 tsp for 1 tsp in original