With a name like "fabricator", you must be technically capable.
If this is the case, I suggest a 6/1 kit engine. I see them on Ebay from time to time. The reason for the kit is personal quality control. The list of problems with my 3 engines is simply staggering. Kit form makes it MUCH easier to prevent such issues.
I feel the larger Listeroids are not as good as similar HP modern engines. But the 6/1 has some very strong points. It is fairly efficient, and burns little fuel by virtue of it's meger output. It is also simple, something the twin is not.
I have a 20/2 and I am neighbors with Rocketboy. I would never do it again. Only a 6/1. These things are crude, poorly made, vibrating monsters.
After 2+ years of tinkering with mine, I am still not done pouring hours into this thing. And, it is not just crappy Indian quality. It is a crude original design too. For instance, I ended up having to make a Heim joint fuel rack linkage, in addition to (what seems like hundreds) of minor modifications/repairs/re-machining to the engine. Then, after all is said and done, the uneven power produced by the engine affects the output of the gen head. My lights all flicker, my UPS's won't work and the microwave groans. Not exactly quality power.
Now, for all the ranting I did, I still love this thing. It is simply the coolest genset around. It sounds great, looks great and always runs.
Chris