This drawing is to scale.
This sentence is perfectly good English. Real publications say stuff like...
The objects have been enlarged ten times, but their distances are to scale. (Astronomy magazine)
The diagram is not to scale: windrows may be more than 300 feet long and up to 10 feet wide, while individual thimbles are about the size of a quarter. (from an article titled "Why jellyfish stick together" in Natural History)
Everything is to scale with him. Many people have long eyelashes; he has lashes as long on the bottom as they are on the top. (Esquire, going on about George Clooney)
(That last example is grammatically flawless, though the writer may not have understood what to scale means.)
The phrase drawn to scale seems to be more common. Both are fine.
Examples are from COCA.
Grammar deep dive
Is to happens all the time in speech and in print. I found a lot of different grammatical situations where it works just fine:
"To alter favour ever is to fear" —Macbeth (infinitive)
The U.S. is to soccer what China is to Procter & Gamble: the last vast, harvestable territory. (analogy)
She is as indifferent to time as it is to her. (weird comparative as ... as construction; it is bizarre to me that something this complicated can be so completely clear and natural to the ear)
Liquor still will be taboo in Disneyland, whose primary appeal is to young children. (the noun appeal in the subject specially licenses the preposition phrase to young children)
The PGA Tour's main responsibility is to its members, which are the players. (ditto for responsibility)
Those don't say anything much about your question, because none of them are grammatically quite like "This map is to scale."
But these are:
When the taste is to your liking, transfer the vinegar to bottles.
In a Newtonian reflector, which inverts the image, south is up and east is to the right.
And -- and all of that is to the benefit of both the north and the south, as well as the region.
In all three of these examples, we have well-worn prepositional phrases starting with to (to your liking, to the right, to the benefit of) being used as predicate nominative. Just like "The map is to scale."
Nobody would bat an eye at any of them. The speaker of the last sentence was then-Senator John Kerry, in 2010.
All the example sentences above, except the Macbeth quote, are from COCA.