I have driven one, R6 swapped. It had a dual range box, so 12 speeds (6 high & 6 low). Top speed was about 115 in 6th/hi. In low it was maybe 75 (6th/low)-both at close to 14,000 RPM. Have also tried to ride it in woods. Useless. The R6 has zero torque down low, below about 6500 RPM is where it starts making power. Even then, you had to keep it mostly wound up which means heat & noise with the noise being an issue (no USFS spark arrestor). Yes it was kind of cool but it was utterly useless for anything but dunes and there ain't any dunes here.
If it were me, I'd find another 660 engine. It ran as long as it did until it blew. The next one you put in, take better care of it and it'll last a long time again. That is, unless you just hate the 660 engine and like to spend lots of money and fiddle with it on a seemingly continual basis.
If you "had" to do a 4 cylinder bike engine, see if you can find an OLDER engine, not so much a crotch rocket style bike, but maybe like an older Seca or something along those lines.....they don't have to run a million RPM to make power, and they run quieter and a lot smoother. etc. Try to stay liquid cooled if possible. Makes things a lot more comfortable since the engine ends up in the middle. That or figure out how to blow air across it and keep it cool without overheating the fuel tank and/or driver & passenger.
Kind of a side story but years ago I had this old Mojave 250, this was back about 1998. I used to drag race it on the dirt drag track locally, with the 250 engine. It was a lot faster than you'd have thought but it wasn't totally stock. Anyway, it broke a valve off and ate up the head, cylinder, piston, etc. I couldn't afford that expensive head so I swapped in a CBR600 engine that I already had, as it was a drag bike. The CBR engine was about twice the weight of the Mojave engine, so the bike's center of gravity changed among many other things. The chassis-as good as a stock Mojave is-was not designed for 120hp. Not even with the Lakota swingarm which is a little longer than the Mojave. It was wheelie prone obviously. Easy fix, wheelie bar and while that fixed it, it's ET's and MPH weren't a lot better than the old 250 was. Sounded cool. I raced it off and on for about 4 years. In about '01 the Raptor 660 came out and people were showing up at the drag strip with them. Torque monsters, and somewhat quick if you could keep the front end down. Well I was paired against a stock 660 in a semi-final round and our dial-in (bracket racing) was within a tenth, which on a dirt drag is pretty close. We both ran our dial-ins he had a better light and took the win. But it struck me odd that the stock 660 was as fast as a 120hp CBR swapped Mojave. Point is, more horsepower means nothing if you can't control it. Later on I bought an '05 Raptor and rode it everywhere, didn't abuse it at all but I also didn't ride it easy. What struck me the most was how well it is balanced (even though it was a little top heavy), how well it would just pull through the woods without a downshift, you could lug it down to about 2000 RPM and just cruise along in 5th at about 25 mph, come up to a hill, just throttle it and go on. I tried to woods ride the CBR/Mojave and it was possible but the engine had to be running pretty hard to make usable power. That engine was never designed to run slow at higher RPM, so it was always running hotter than I liked, and made a lot of noise. I ended up selling it to a friend who continued to race it until it blew, then he put a KLR650 engine on it (to be like the Raptor boys) and to my knowledge still has it, probably laying out in the weeds.