[@necroKnight ]

Not as much as you think, apart from these things being built to be resistant to the elements. Yes the finer points of gun stabilizers and infrared sights might need re-calibration or repair. The tank will still move, shoot and be a tank though. 

Clean and flush the engine, sand off rust and put fuel in it. Unless it literally sat in a bog for four centuries....it’ll work 

For comparison, the majority of the aircraft and ships  that militaries from around the world use are at least 30-40 years old 


The battleship your using ( unless it’s brand new) would be a refit if either a British battleship circa 1945 or American also circa 1945 buy modernised to 1990s tech