yamaha fzx

hello to all,been out of bike scene for 25 years and would like help and guidence to bring this bike back to some sort of life,as it has been used,abused, bodged and neglected.Just bought 1987 yamaha fzx 750 with a 1000cc genesis engine?,if i want to replace engine but stick with 1000cc am i looking for an fzr engine,if so what kind of engine years am i looking for to fit standard frame.Im trying not to do any major mods to fit engine.Any one done this,all help,guidence or info from any of your good selves would be very much appreciated.Looking for appropriate engine anyone got engine for sale,has to be good,and 4into2 down pipes.Thanks very much for your time,heres to hope