Any suggestions for the best drive chain for the DT360, preferably ‘O’ ring if one will fit.

Hello John,
"O" ring chains are great because they reduce the dirt and grit that can wear a chain out . However, an o-ring chain will rob some HP from your engine . The DT360 engines do not develop that much HP compared to much later engines that o-ring chains are used on. Due to HP loss, which would be noticed, I would recommend that on these older machines you should use a non o-ring chain.

I'm not sure specifically on the DT360, but most o-ring chains are too wide and rub the cases (or chain guide or something else) on vintage bikes.  DID makes a narrow x-ring chain that I have used....  I think it's the "VT" series.  It's also supposed to be low friction as DVM's point is valid.  

dirtbikemagazine.com/did-520-vt2-x-ring-chain-product-review/

Might be called the ERVT these days?  www.didchain.com/collections/didchains/products/520ervt

While picking up parts for my CRF450R @Honda Dealer, bought one of their non o-ring 120 link chains. I always get the long one and cut it for my bikes.. It barely clears the frame near rear sprocket. I need to cross check alignment and possibly space, but recommend keep it simple for this bike.