I'm in the UK with 50 Hz mains. We used to run a converted 260 cid Ford inline six on landfill gas. It made 60 kW max at 1500RPM and ran 24/7/52 for almost 4 years (33000 hours). It only got removed because we installed a much bigger unit. I'd guess you would probably need something around the 120 cid, 2 Litre mark. A converted Gasoline set would be a good option. The biggest problem I could foresee is mating it up with the alternator. If you can get hold of a gasoline generator of about the right size, most of the work is already done. The next biggest problem is the governor system. Again, this is side-stepped by using a gasoline generator. Our set was a Diesel engine, converted to spark ignition. I assume this was at least partly because it was easy to mate the engine to an alternator with off-the-shelf parts.
I know Volkswagen were selling small industrial diesel engines quite aggressively in Europe a few years back. I think they were basically Golf/Rabbit engines so the bits needed to fit an alternator may be available. I can't be any help on US domestic engines, I'm afraid.
Next problem is the carburettor or gas mixer. Our Ford set used an Impco for a while and it worked pretty well. It did not cope well with the varying gas quality from the landfill, so was changed at around 20000 hours for a too-complicated system. We had a couple of converted Deutz engines running on landfill gas, too. These had the most elegant and effective gas mixer I have ever seen. Armed with a small lathe, one could be made in a weekend by the average home machinist. I have no drawings or photos but could scan and email a (very) rough sketch, if you are interested.
You will need to check out your local requirements for gas safety systems. Here we have to fit dual, approved solenoid valves in the supply line. Above some line size, a test unit needs to be used which opens the upstream valve, closes it, waits a few seconds, checks there is still pressure between the valves, opens the downstream valve, closes it, waits a few more seconds and then checks there is no pressure between the valves. If all is well, it opens both valves and feeds gas to the engine.
You may find the cost of the gas supply train and its control are the biggest obstacle you face, especially if you have to buy new.
It is worth checking the gas spec with your supply company. Particularly the limits on "Methane Number". This is a measure of the detonation resistance of gaseous fuels, just as Octane Number is for liquid fuels. Gas from different sources can have very different methane numbers. In the UK, we have North Sea gas, formed at high temperature and almost pure methane, Methane Number near 100. When this runs out, we will need to import gas from wherever we can get it. I have seen specs for Eastern European Natural Gas with a Methane Number as low as 45. Formed at lower temperatures, this has significant amounts of Nitrogen, ethane and ethylene. It probably also has some propane and perhaps some butane in. Generators optimized for North Sea gas are likely to start falling off their perches if fed with something with a MN of 45. Realistically, it's not likely to be a problem, but I'd try to cover all the bases before taking on a big project.