OP still around? What exactly is the "THD" problem he is facing? Is it a very old existing genny, built before the current crop of open-frame gennies (a few years back)? Is he fighting the hype of "THD is bad for everything in the house"? HVAC will die if a high-thd genny is used?
Over on PowerEquipmentForum, we debate THD stuff pretty heavily; from everything I can gather, it's more a marketing-driven FUD tactic, than a "real" problem. HVAC control boards will die from many other common things in your power system (power surges chief among them), long before supposed THD gets them. Things in your house (UPS's, switching power supplies, etc.) will throw off their own THD back into your power system, and so on (it's been this way for years, now).
Not saying you shouldn't buy into the THD FUD, as what it does well is drive the sales of very expensive/complex gennies, with less power than their open-frame counterparts; this is good for the economy overall (in dollar sales)!
But, in the open-frame genny world, it is the type of stator/rotor (mecce-alta is the best), and/or the vendors' own choices in parts, that make for a lower THD genny; most recent years' gennies meet this trend (whether or not they report THD). Couple that with how you operate it, as your operating procedures could increase THD (variable loads, running at/near max) regardless of reported THD numbers.
In my own case, I use a 12%THD duromax propane 12kw ... for years now ... no THD problems; in my case, it just drives an inverter/charger (at a steady 50% or less, 240v load) to refill the battery bank, so in reality it throws very little THD at all. I'm over 5 years now, and approaching 10, with a line of open-frame gennies (generac, duromax), and never a THD problem. No devices dying in the household.
Also consider that there *are* open-frame gennies, like the westinghouse "Wpro" line, with very low THD numbers if that is your highest weighting among tradeoffs, at much less cost/complexity than an inverter/generator ... and others, with some careful shopping, if you still want a low THD number.
Inverter/generators have higher costs, lower kw numbers, more expensive parts inside them, and complexity that looks like a jet engine (packed in tight). I couldn't figure out how to support/service them in my rural area (also why the Generac had to go).
As best I can tell, after lots of research, I've found 3 use cases where THD does need to be solved, with EE's getting involved ... factory complexes (weird motors), commercial buildings (lots of pc's and others), and recently, movie lighting suppliers (with massive lights & ballasts). At the home level ... zip; lots of FUD at the home level, but zip on facts.
Even with all the research, and absolutely nothing pointing at THD from gennies being bad for our households, there are still plenty who will say "nothing touches my home but an inverter/generator" ... more power to you (in a sense), as it is just one choice among many trade-offs to make (fuel type, KW, complexity, etc.)!
Hope this helps ...