rebuilding 2005 rincon 650 to 680

i just purchased 2005 rincon that needs some attention. lots of smoke when first starting then goes away. smells like oil. was looking on here and most people are saying to rebuild to 680. is that reasonable? is connecting rod same? do i need to buy new head with cyl and piston/rings and wrist pin bearing. I have done 2 strokes but never have something like this. also this wheeler likes to pop out of reverse. and kid put a conversion kit on front rack for coolant and discontinued the thermostat. i have had alot of hondas and figured this wouldnt be a hug issue... am i over my head to try and restore this unit to the trails? thanks Jim