I'm not on the forum a lot any more but have built a few of these in the past.
Bitzz is correct that you can't get CPI's that have a 68mm bore and 58mm stroke with the power valves any more. Calvin stopped making those back in 2014 as the didn't sell as there is less benefits with the valves on bigger standard cub motors
You can however still get 65x54, 68x54 and 66x58 Cheetah configurations with valves . That will give you engine sizes of 358, 392 and 396.
In theory you could take the 68x54 and use it with a long stroke crank and let the piston run 1mm into the head the way the long stroke RZ motors work. I think the problem is that the top of the exhaust port on that cylinder is 27mm from the deck, while the cylinder designed for the long stroke motor is 28.5mm. I had though about this but the 27mm will give you port timing of way over 200 degrees which is not good.
Pipes do make a big difference on these bikes. Off the shelf pipes like Lomas or Jolly's are not great with the Cheetah's or Athena's triple exhaust port so I would avoid those, but you can get pipes made for them.
Mutts (Dave Whattam) and TSA (Kenny) both do really good pipes for them and being from the UK have probably already built them for a Mito chassis. They also do different versions so you can get a mid-range pipe, all round power pipe or something that is more for top end
The other guys would be Roger (Wicked), Matt Shearer or Brian Turfrey.
It might be harder to get them to build a set of pipes to fit the Mito chassis so you may have to do some fabrication yourself to get it to fit. Matt makes these pipes all day long for 421 Banshees. I had a set on a bike years ago and did like them. Brian has been in the business since the 70's and he built the pipes for Kenny Roberts when he was riding and when he ran his own team so he knows his stuff.
I personally think the 421 can be quite streetable with the right setup.
My "Daily Driver" is a 421 Cheetah cub (no valves) with a cleanup port, a set of Mutts all round power pipes and some PWK35's. I'll dig out the dyno chart but the power is very linear and doesn't have that big hole at around 5k that you tend to get with the standard RZ. The maximum HP is 82 around 9900 which is good for crank life.
I'm also racing one in AHRMA at the moment. Its a ported Cheetah Cub 421 with a set of PWK39's and using a set of Turfrey pipes so there is very little until 7k and then it just revs to the limiter at 11k, so there are a number of ways to build these things depending on how you want to use it.
The Athena cylinders are fine, but I would use a different head. The oring groove tends to be very thin and they chip and crumble. The CPI Cheetahs are definitely better.
I think the problem might be getting any cylinders at the moment as most suppliers are out of stock and I know CPI are having problems getting the castings done because of COVID.
I'm building a 396 cub at the moment 66 bore x 58 stroke to go into a 350LC chassis that I picked up, and it was the only block I could find. I just put the motor together over Christmas and picked the pipes up last week from Brian
Oh, just one more thing. If I do a Cheetah I like to use Banshee Crank Cases. They are pretty much identical to the RZ and are completely interchangeable, however they leave a little more meat on the crankcase where the front of the cylinder sits so its less likely to leak from the water plug on the bottom of the cylinder.
And one, one more thing. You can actually get a head that takes the standard RZ350 thermostat and housing, or you can use and in line one from a KTM enduro bike. It makes a difference on a street bike. I have the KTM model number somewhere
The one I'm currently building is below (sorry. I just noticed those pictures were huge. I uploaded smaller ones)
Feel free to ask questions about any of this
