Col. Its just a copper tube in a hole. How they did it in the factory does not matter, all it needs to do is stay in the hole. So, on plunger engines sometimes the pipe ran down the back of the crankcase, secured by a P clip on the through crankcase stud. This meant all the oil drips from the sump plate joined those from the breather. Some Plunger engines have a very small pipe which leads back, secured under the the part of the casting covering the chain, effectively dropping the drips on the chain to do some good. Originally the pipe was were sealed in, probably with some sort of varnish. Dip in the glue, knock it in, bend, bolt, next....
You just need some thin wall tube which fits nice and snug in the hole, sealing and securing with modern Loctite, even epoxy or cyanoacrylate glue will also be fine, if you want true perfection.
Where it goes is up to you, but leading it down to the rear of the gearbox sprocket, to oil the inner side of the chain, is a good plan. Make sure it is well supported.
The excellent pictures above show attention to detail that I can only envy, so aim for something like that.
Thinking about S/A motors I reckon most are just a hole....at least it stops the chain guard joint squeaking, and prevents rust....another masterpiece of cr*p design.
Swarfy.
