Either one is a good engine. The HKS does great in a light aircraft, but a heavy one, not so much. It won't do what the 582 did in the Avid. The Jab would be more suited for the job in my opinion. Before I spent the money on the HKS, I'd talk to someone who flys one in a similar aircraft, or even a road trip to fly in it or get firsthand info and look at the installation. Even being a four stroke, if it doesn't do what you want, then you're back to where you started only with less money. The Jab, you can get honest information here on the forum. They have the actual real time experience and give you the truth.
I would say stick to what works, but if we did that, we'd all be flying the "Wright Flyer" still. One would probably be shocked to find out where some homebuilder ideas are being used today! I can proudly say we homebuilders aren't satisfied with anything!
Take the 2K and rebuild the 2 stroke every 2 or 3 years and you have about as much reliability as one can expect from ANY engine. Even every 5 years ups the scale a bunch. Unless one flys a couple hundred hours a year, a 1500-2000 hr TBO doesn't really mean a whole lot. Most average rec pilots may get 50 hrs a year so 20-30 years down the road, rubber engine seals are already done in. And at the price of a four stroke overhaul, one keeps on flying a ticking time bomb till it goes boom. A lot of 912's on the market for reasonable prices and low hours, but how old are they? Does one want to fly a newly overhauled 2 stroke or fly a "seems to run alright" engine that's X # of years old and never been torn apart? Everybody has their opinion. Not saying anything but want everybody to consider the amount of risk they are willing to accept vs. cost.
New, most 4 strokes are in the 15K-20K range. A couple under that but generally in that range. A tad too much to put on a classic airframe such as an Avid/Fox, at least for me anyway. Especially when a 2 stroke can be had for under 10K plus a few hours learning how to setup and operate one. Those working on the Apex or Phazer engine conversion might be on to something. I think there are more possibilities out there.
A few options, foam carved using a hot wire setup, remake new wood fairings, make fiberglass ones, etc. Only ones I know of that you can buy off the shelf are the Kitfox ones. QCU Challenger did sell plastic fairings for 1" tubes for around $125 in 10' lengths. Welcome to the world of homebuilding. Sometimes buying isn't an option. Fabricating something is the only option.
I've looked at the UL 97 HP but its a little heavy for a model 2. Still lighter and cheaper than a 912. Weight wise the Jabiru 2200 is one of the few viable 4 cyl, 4 strokes available that will work in a classic Avid/Fox. The HKS is very close to a 503 in performance even though its a four stroke. But parts and service might be an issue. We have run this engine thing all the way around the barn and still end up back where we started. Probably more Avid/Foxes flying with a 582 than all others combined. For the best 4 stroke, the jury is still out. And most likely will be for a while. Been that way since day one of experimentals.
Seems to me when considering all options that the simplest way is to loose weight off the airframe and go with a lighter HP engine with less HP. Nobody likes to give up their goodies that they think they need but dropping airframe weight is a lot cheaper than more HP engines. If I remember correctly, the original Avid/Foxes flew just fine on a 50 HP 503. In that frame of reference, a HKS would do just fine. I am as guilty as anyone, but if we worked as hard to drop airframe weight as we do to shoehorn a bigger engine into the nose, we'd probably be better off and have just as good of a performing plane. But sometimes my opinions are not the best, just ask my wife! I am going to stay with my little 55 HP Hirth till I learn better.
I've always wanted to build one, but it is a time consuming build and not an easy build. But one could make it into whatever type of plane he wanted. At this stage in my life, a folding wing bird is the only choice and not too many types of those available. Doesn't look like Avid is coming back, A new fox is out of reach, so this old classic model 2 fits me the best.
Its generally hard to beat a set of torches. But any setup isn't worth a dang unless you can use it. One project or a small job. its more about how much you want to invest and the learning curve to go with it. At my stage in life, I'll stay with my torches and take the special welds to a welder. My little mini Smith torch is perfect for 4130 tubing.