First off, you said; "it is an v6 inline". Nope!
It's either a V-6 (3 cylinders per side) or it's an in-line with in-line meaning all 6 cylinders are in a row. I suspect it's an in-line.
Anyway, yes....finding good help to repair an old 2 stroke Detroit gets harder every day as all the old timers drop off the planet. I haven't worked on one in many years and, even though I still have most of the special tools and manuals, my recollection and skills have drastically faded. But IF you have the CORRECT engine manual and are willing to read it, these engines can be repaired by anyone with reasonable mechanical ability. Detroit manuals are very thorough....as engine manuals go.
Answering your question about if the oil can get in the coolant via the head gasket, the answer is YES if it's an inline engine.
And probably NO if it's a V-6. The V block engines don't use one piece head gaskets.
The oil intrudes via the one piece gasket if the seal between an oil passage and a coolant passage is compromised. And, just as I explained about the "reverse leakage" in the cooler taking place after engine shut down, the same can occur with a failed head gasket.
There's no reason to monitor the oil sump to block for coolant leaking. It can't happen. I recommended draining the oil and leaving the drain plug(s) out to see if coolant is entering the sump.
In a shop setting you would use regulated shop air to test as it will keep your 15 or so psi on the system without leaking down as long as you want. If you're just using a hand pump it leaks off pressure too swiftly so you will want to pump it up several times in order to detect internal coolant loss.
This applies to watching the air box inspection holes as well.
Taking the head off is an opportunity to do a complete repair on one of these "modular" engines and is something that should always be considered. If the engine is high hour, low compression or unknown internal condition, this is where you would, ideally, choose to do a "semi-overhaul".
With the head removed the pistons and liners can be replaced along with new rod and main bearings. In the long run, this could be a labor and cost saving way to go but might not be very attractive to a diy'er. Lots of measuring and additional special tooling required. Not to mention additional parts cost. Just sayin'.
As far as finding major damage, as in a cracked head or engine block...
Most folks these days choose to repower with a modern platform since the costs can get up there in finding and purchasing these replacement parts.
As far as cleaning up the cooling system, there are commercial degreasing and flushing solutions available. I have also used Tide detergent and Dawn dishwashing liquid in my "storied" past. I didn't like the Tide as it was so "sudsy" that it was difficult to get IT out. Makes a big mess too that wouldn't be very welcome around some marinas these days.
Before I retired, I experimented with a microbe based cleaner called Oil Eaters that was very effective and easy to clean up. It actually seemed to work like magic! Too good to be true sort of magic! But...that was several years ago so I'm not sure if that option still exists.
One last question:
Are you taking heat from this engine into the house for any reason? I know some set-ups "T" off coolant supply and return lines, usually through an electric "diversion pump", to provide hot water for space heaters or windscreen defrosters. If so equipped you have to also consider that for coolant loss.
Good luck with all this.
as oil in the coolant sounds like there could be a serious issue. I'm not exactly sure what that issue is. Educate me if you are inclined, please and thank you.
