Looking at the spark plugs, I would say that the one on the left looks like it has a bit of residue on it but not real bad. It is hard to tell without feeling of the end of the plug if it is fuel residue or oil. If the residue is fuel it will be pretty much dry and feel like powered soot, if it is from oil it will feel wet and slick. The plug on the right looks pretty good but may be running a little lean due to an air leak from that funky two piece manifold or the missing manifold to block gasket. While you have the manifold off look it over carefully for air leaks and for good measure put a small bead of sealer on all of the seams. That funky two piece manifold was not one of Onan's greatest ideas. The left plug might be slightly fouled from running a richer mixture used to help compensate for an air leak on the other side.
Be prepared for a shock when you price parts as they are quite high. Some parts like coils and point sets are available from Amazon and the auto part houses. I bought a coil for mine from Amazon last month. Some important words of caution--- DO NOT check for spark on one plug wire with the other not grounded or hooked to the plug as you can burn up the coil doing this. This is a waste spark firing system because both of the cylinders fire at the same time with one on the power stroke and the other one on the exhaust stroke, thus the name waste spark ignition. I have an 8Kw belt driven generator with an NH series Onan that had been on a tire service truck air compressor before I got it and I have used it for more than 30 years myself. It has been very dependable and almost trouble free other than a carburetor kit, spark plugs, points and finally last month a coil. I have dealt with Onans for many years and most are good but a few were bad news. Some guy on this YT site recently took one out of a Case skid steer and replaced it with a kubota diesel.
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Today's Featured Article - Restoration Story: Fordson Major - by Anthony West. George bought his Fordson Major from a an implement sale about 18 years ago for £200.00 (UK). There is no known history regarding its origins or what service it had done, but the following work was undertaken alone to bring it up to show standard. From the engine number, it was found that this Major was produced late 1946. It was almost complete but had various parts that would definitely need replacing.
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