I put kerosine in an old spray bottle and use this and rags to clean painted or metal surfaces - old sox are great for getting in nooks and crannies. I put my hand in the sock - after it is dirty, i put a clean sock on my hand and turn the dirty one inside out to use the clean inner surface.

Kerosine dissolves almost all gunk and leaves a residue that fights rust.

Foe plastic and rubber, I use silicone spray that is safe for plastic. Silicone IS safe for plastic and rubber, but cheaper spray cans use a propellant that is harmful to plastic and rubber, and you will not see those materials on the used for list.

This also leaves a residue that fights rust - once and a while, I spray it on mechanicals that look dry..

I take ignition parts off to clean them.

These methods take time, but do no damage. I think that there are many items in an old car that can be damaged by water and detergent that you cannot remove properly- for example, the pivots in the throttle linkage and old fashioned cables

When I got my '64 544, it was VERY dirty as the original engine had worn out and spewed dirty oil vapor over everything for years. It took quite a while just to clean the inside of the hood!