What degree of restoration does a tool need? Does it just need a clean, or are you going to sandblast every speck of rust and residual japanning off, repaint it (or re-japan it), and painstakingly restore every single component, sourcing original parts for those that are missing. The degree of restoration depends partially on the state of the tool, the amount of effort to be put into restoring it, the quality of the end result, and whether the tool is a collectible or a user. For example, using an authentic “Pontypool Asphaltum” formulation for re-Japanning a plane is to be commended, but it won’t make that much difference to a workshop plane – Dupli-Color Engine Enamel seems to work just as well.
If a plane has a considerable amount of its original Japanning/enamel, and just some surface rust, then there is very little restoration that needs to be done. Some tools are naturally beyond the point of being restored. The body of the Stanley No.103 below is considerably corroded, has extensive pitting, and a seized up depth adjustment mechanism (this mechanism can’t easily be taken apart). The rust can certainly be removed, but the pitting has caused irreparable damage to almost every surface of the plane. Considering how common the plane is, it’s not really worth saving.
In between, there is the gamut of restoration. With some tools, restoring it *may* be a challenge. It could be that the tool has been sitting in a barn for the last 50 years, and requires extensive restoration – it may be heavily rusted, have little of the original Japanning left, parts broken or missing, or a worst case scenario – it has a broken casting. In most cases the tool can be restored (except for the broken casting) – but it will require rust removal (chemical, abrasive, electrolytic), restoring the finish (remove the Japanning and replace with paint, or new Japanning), lapping the sole, refinishing parts (e.g. handles), replacing missing or broken parts (or fixing them), and sharpening.
If the tool has parts missing, there are certainly places for obtaining some original parts, for example Stanley parts are relatively commonplace, but others may be extremely challenging to find. The Millers Falls No.714 “Buck Rogers” jack plane is an aesthetically beautiful plane – but its challenging to buy parts like Tenite handles, and even replacing the blade takes away from the aesthetic value of the plane.
Anything is possible, given time and perseverance.