HI I have only flown the 582 and 618 in an X-air. There's limited info around on the differences between the rotax and jab engines in X-airs.
Xair Standard and F Models - ByDanJohnson.com
Xair Ireland Web Site - Xair Engine Options
Xair Peformance - Xairuk.com - The Wessex Light Aeroplane Company
Make sure to read this one especially the second article
X-Air Australia - XAIR Ultralight Aircraft
I'm in the same boat as you with an aging 618 (400 hrs) that is going fine and being used for about 90 hrs per year. For me personally I will probably just go with a rebuild on the 618 from Rotax Rick ( Pricing) for under $A 5000 after freight. It should then be good for 450 to 500 hours. I reckon about $10 an hour for a motor is pretty good.
I too have looked at 4 stroke options. The Jab would be nice I guess but there is no guarantee it will last any longer than a rebuilt 618. It also requires new engine mounts, cables and instruments.... All in a New Jab 2200 conversion is near $19K. Even if it runs for 1000 hrs it still works out almost twice as much per hour as a rebuilt 618. Sure 2 stroke oil costs, the 618 burns a couple of liters of fuel extra and I don't like Rotax prices but there is a big difference between $5K and $19K on an aircraft that would be lucky to sell for $14K.
There is also an interesting alternative if you are set on a new 4 stroke.... Aeromomentum ( Aeromomentum Aircraft Engine AM10 ) probably less than $13K all in conversion , fuel injected, light and 1500 hr TBO.
Good luck, please don't think I'm any expert on this one... I'm hoping to make the best choice too