I forgot about the D-R-N-M1937; in any case we should retcon it to detect submarine
conning towers, not periscopes, as radars still couldn't detect things measurable in less than meters across.
In any case, the cavity magnetron was a product of years of intense, sustained basic science research; if we want to be able to actually invent new stuff instead of just engineering with pre-existing components, we'll need one or more well staffed dedicated research centers - and years of work - on each project.
How long? I'd say that each basic research project should yield results in 3-5 years or even more, depending on project difficulty, staffing, equipment, morale, funding, political meddling (
political appointees, commissars and pogroms tend to be detrimental to scientific research) and enemy action (be it sabotage, commando raids, or outright carpet bombing of the research centers):
-the cavity magnetron itself,
-atmospheric jet engines (which are different from the self-contained rocket engines),
-vehicle-portable remote control should also be feasible ahead of time as live TV broadcasting already existed (remember, back then spies needed a large suitcase to carry even their "miniature" morse-keyed valve radio sets),
-magnetic recording tapes should be easy enough, as the Germans already developed (top secret) audio tapes from magnetic wire technology; it's only a small(ish) cognitive jump to also start working in applying the medium to video and data (VCRs and getting rid of IBM's punched cards decades ahead),
-the transistor is still a decade away, as it requires previous work on semiconductors (and hard enough to merit a Nobel prize),
-magnetic propulsion, leading to both railgun and maglev researches,
-magnetohydropropulsion (from "the hunt for the Red October") is based on the previous one, but still in the realm of sci-fi (or maybe not? It's not
too outlandish not to have been at least attempted by
someone, and it might very well still be a secret if it actually worked),
-or even genetically modified cyborg sharks with laser death rays on their heads (to which we could add some ninja training, and... no, never mind).