Whats best to use to flush an engine in unknown condition,

it is not driveable/unknown/ it could be seized?

Is diesel or CRC/WD40 possible to use in a mini engine?

First check the oil, if it looks black & sludgy, change it and the filter first.
Next, pull the spark plugs out, check if engine can turn over by hand. If so, squirt a little engine oil into each cylinder.
If not, I would pull the head off for a look.

If standing for more than 6 months I would prime the oil pump before trying to start it:
Remove oil pipe banjo bolt from block,
squirt motor oil into the block until hole is full,
with 4th gear engaged, push car backwards for 2 revolutions of the fan,
squirt more oil in, refit bolt but leave 1 turn loose to let air out.
Select neutral.
Crank engine for no more than 20 secs with plugs out. If oil flows out around the bolt, good news- you have oil pressure, tighten it up.
If it just gives 1 burp (what oil you put in) and no oil flow, I am sorry but the oil pickup strainer is plugged with sludge. Engine strip time....

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

No oil is in engine or gearbox/sump.

It has been sitting for more than six months, it could have been years,
the engine flush I have looked at, appears to want the engine running and run but as stated it has been sitting for a long time.

I believe I need to lubricate bores and rings as you stated,
I have heard of people using diesel to flush, but unsure.

I might try oil with millilitres of engine flush,

I have not heard of anything else/product to clear sludge from gearbox pickup/strainer,
this is why I suggested diesel, but unsure if diesel will damage bearings and seals in gearbox or piston rings/bores in the engine?

Forget engine flush, fill with 20w50 oil then follow what I said above.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I've had good results using ATF in the spark plug holes & letting sit for a week for so after Dr's advice has been carried out to help free up stuck pistons but if it's to that point a rebuild will more than likely be needed.

Brad

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During the day I work on English designed cars & on my own time I work on English designed cars....