Who wants to clean my pistons..

[Niall]

If the piston rings are getting changed anyway does it need a replacement engine?

If you can get a known good engine, it would be quicker to swap the engine than it would to change the piston rings and valve guide seals. If it were me, id buy a known good motor, drop that in as a short term thing and then keep the motor you currently have and rebuild it. If your whipping the pistons out to do rings, you should really do the big ends as well (don't ask me to do them!) and it all adds up quickly.