Can I use denatured alcohol instead of isopropyl alcohol when servicing a fork?
Yes, denatured is fine to substitute for isopropyl. For regular shop use, the methanol is kinda nasty (debatably) from a toxicity/odor standpoint.

Everclear (which is 95% ethanol) is the other thing from around the house that can sub in.

Isopropyl is more a drug or grocery store item, although industrial suppliers do sell huge jugs of it.