There is a services manual for the engine , manual cover all early diesel engine .       

An RD-6 Service manual is about close as to the 35
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thankfully these old girls are pretty simple. and have very few problems. you should have much problem driveline wise provided the clutch lever was always left enguaged this stops the clutch plates warping. thier is a great primer on reviving a static unit on the forum. it has a lot of handy hints for getting a old machine moving again.

the first thing i would do is remove the manifolds and check for debris and signs of water this can help decide if the engines need to be stripped down or if the might free up.
then try to turn the main with the engine decompressed the lever need to be at the start position this lever will be near the throttle and choke for the pilot engine. if it turns over then no major problems you can try and to start it. its a good idea to check the main engine governor lever moves freely it does have a ratchet feel but check for full travel.
drain off some fuel from the tank looking for water if none found then its okay unless really dirty. same check the engine oil for water.

then you can try and tow start it.

and there is a operator book in the tech library. i just found it by searching for 6e. there's a parts book too.