Without knowing the model, the history, and the condition of the engine, it`s impossible to comment accurately and compare the Rotax 503 to the 582, not only that! you need to know the aircraft ( single or twin seat ) and performance expected from the aircraft.
503 versus 582.
503: Is it single carb, single ignition or dual carb, dual ignition, new or used, if used how many hrs has it done, what type of gearbox and prop is it driving? The 503 is fan, air-cooled, it has fewer working parts than the 582, grey or blue head, oil injected or not, therefore, fewer things to go wrong.
582: is it grey or blue head, oil injected, new or used; if used, how many hrs has it done; what type of gearbox and prop is it driving? The 582 is water cooled, therefore, it requires a radiator and all the things that go with that, also, Rotax now recommend that the Crankshaft be replaced every 300 hrs.
Hopefully! I`ve outlined some of the questions that, need to be answered, to be able to make an informed decision.
I`m on my second 503 and I went through four grey head 582, purchased new, when I was instructing...I`m currently running the 503 DCDI, purchased new, with the E-type box turning a 60-inch ground adjustable, 3 blades, Ivo prop, on the certified twin seat Austflight Drifter! from memory, the engine has done close to 500 hrs and so far it hasn`t missed a beat but I`ve fitted new rings and gaskets twice and all the parts in the carbies, once.
At the moment, parts for the 503 are still available from Bert Flood Imports, Pty Ltd.
Franco.