I would stick with the Gardner though I am unfamiliar with it , but from the description it sounds easily serviceable with the heads separated into 2 individual banks .
GARDNER DIESEL ENGINE – 8LXB
Gardner Diesel Engine – 8LXB
Engine Specification
Marine – 175bhp@1500rpm
Automotive – 240bhp@1850rpm
https://gardnermarine.com/engines/gardner-diesel-engine-8lxb/
What are the symptoms that leads you to believe it needs rebuilding , smoking or ticking or knocking ? Displacement and horsepower ? Age and how many hours?
I'll give you a little of my mechanical background with my inline 6 14liter Electronic injected and turbocharged Cummins that is based on the previous mechanical injected 855 big cam Cummins believed to be the most reliable otr diesel out there as your Gardner is in the marine world .
My over the road semi truck now has 1,275,000 miles on the original motor. At a average of 40mph when running under load the motor has 31,875 marine diesel hours on it with another roughly 5,000 hours of idling on it for a total of 36,875 hours.
At 975,000 I had the heads resurfaced and rebuilt with new seat and valves and springs because I was getting some oil smoke on start up that indicates worn valve guides .A couple thousand dollars in parts and my labor, very easy to remove and replace being they are 3 separate manageable heads comparable to your 2.
The heads have the fast moving parts and sealing surfaces that wear out long before the piston and bearings on the rods and crank .
In the trucker world we do whats called a bearing roll in at around 750thousand miles that simply involves dropping the oil,pan and loosening the piston rod and block main bearing caps one at a time and slowly roll out or push the 2 part bearing and roll or push in the new 2 part bearing . If you can get access to removing your oil pan with out lifting your engine this should be a fairly simple process done in a couple days at most.
If it needs new pistons or rods that is a little more involved but still pushing out the old piston and liner is not rocket science , I would much rather rebuild 3 wet liner low rpm diesels than I would 1 unlined gas motor .