Is it possible to tag all loot which can be used in the schematics?

While Fallout 3 only has seven crafting recipes, the items can be created repeatedly for profit. It can be difficult to remember 20 or so items you may want to keep an eye on at all times while roaming the wasteland.

To my knowledge the game doesn't mark these parts — a mod is an acceptable answer.

From what I can search, there are no mods which can be used to tag items used for crafting. If you haven't already, try searching NexusMods for item tagging, crafting or similar. You might have better luck in your search than I did.

I can confirm, however, that there is no way to tag items for search in the base game.

I can offer some advice regarding searching for these items, however: When you find one item used for crafting you can often find several other similar parts in the vicinity.

For instance, near Springvale School you can find a motorcycle gas tank and motorcycle handle outside the school, used for crafting a Shishkebab sword. Nearby, at Super-Duper Mart you can find both a Steam Gauge Assembly and a Crutch in the building, which are parts of the Railway Rifle.

It is not necessary to remember all parts used in crafting. If you can identify one part of a craftable weapon you'll often be able to find related parts scattered nearby.