What do you think pistons,rings(or just rings?), valves?
Depends on what your long-range plans are - how long do you plan to keep?
An 8400 hour motor will probably be tired and will need a decent amount of work done. To really answer your question you need to disassemble and mic all the important parts - cylinder bores, pistons, crank journals (mains and rod journals) valvestem clearance, head and deck flatness, cam, oil pump clearances, etc.
If you want to get decent hours (many thousands) out of the rebuilt motor as a
minimum you will probably have to:
- Bore cylinders 0.5mm oversize
- new pistons/rings
- check/replace conrod small-end bearings
- valve job (grind/replace valves, grind seats)
- Check/replace valve guides as required
- Main bearings, rod bearings
- check or replace oil pump (I always replace - good insurance!)
- new water pump/thermostat/radiator cap
- all the usual maintenance stuff - plugs, belt, hoses, filters, etc.
- complete gaskets/seals kit
And unless you want to be constantly plagued by maintanance/petty breakdowns, now is the time to proactively replace/rebuild 8400 hour parts that are guaranteed to cause you problems down the road, like rebuilding or replacing starter, alternator, carb, etc.
On the other hand, if this will be a short timer, you might do little to nothing and get another thousand or two hours out of it, who knows?
I think it's worth it to have all the disassembled parts checked for wear before making a decision - if everything is super loose then slapping new rings in it will do little to extend the life of what you already have.....