Replacement Engine

I own an 82 KZ550 LTD and I'm wondering what other engines might fit on the frame (old or modern). The reason being that the orig engine is wearing out fast and I would like to take it and the gearbox off to rebuild them over the next year or two (don't have lots of time on my hands) and was wanting to replace it for the time being. Eventually I want the replacement engine to become my backup incase I ever damage the orig.

Make/Model doesn't matter to me. The only thing I've wanted was to either upgrade it (even if it is temp) to a bike with an oil/water cooler and/or one thats a little bigger. Eventually (years down the road) whenever I have more time/money I want to attempt to build a bike and will use the replacement engine for it.

Thanks for any advice/help that is given.

You could probably get an entire cheap replacement bike for temporary use for less money and headaches. Just a thought.