Not a lemon at all.
There’s nothing wrong with the older ignition, carb, power valve and airbox (the square airbox actually has a performance advantage over the one it was replaced by) unless you are wanting to get into MSA level racing. For the endurance racing I did, the older spec was written into the rules as there is less to go wrong and they are cheaper to buy and maintain. You just need to feed in the throttle as they don't like being planted which is one of the things the later Evo upgrade sorted. It’s hard to tell just by looking at photos but it doesn’t look like it’s been set up to race as it doesn’t have the extra seat braces you would use to force load into the rear bearing carriers and this is a good thing - raced karts take far more abuse than one used for the occasional track day/test session – there will be less collision damage and there is likely to be far less flattening of the tubes underneath from running kerbs.
With race gearing, the engines will go through a new piston and ring every fifteen hours (some racers will change this every meeting) and a new rod 25 to 30 hours. For test days you can reduce the rear sprocket size by something like three teeth and reduce the stress within the engine to extend the life of the rod. As you’ve identified, these engines are jag sealed and a local builder will happily charge you to replace the internals and set the squish to a setting that he finds works best. If you’re not racing you don’t need the seal so you can do the rebuilds yourself and keep the costs down although some of the engine builders have very reasonable costs. If you don’t know how many hours are on the engine since last rebuild you can get an indication by looking at blow-by below the piston ring by looking up the exhaust port (obviously this doesn’t tell you what the rod is like). These older spec engines can be bought for peanuts now so you can just run yours until the rod lets go and get another. If you look at the barrel it will have a number or a two letter code cast into it - they did change over the years and some work better than others. Do you know when it was last run as if stood with fuel in the carb you might needs to clear the idle circuit jets out - there's some tiny orifices that get blocked and make the engine bog out of slow corners.
If you’ve only ever done arrive and drive karting the rotax will blow your mind. These things accelerate quicker, have a much higher top speed and will corner significantly faster than what you are used to. Make sure the seat is the right size for you and get hold of a rib protector or you will bruise or break ribs.
I’ve been karting for over twenty years now in most of the available classes - Prokarts (just did the British 24hr a couple of weeks ago), Rotax Max, 125, 210 & 250 gearbox (often referred to as ‘National’), Super Pro (600cc V Twin) and Aixro (50hp Wankel) and can honestly say that the Rotax Max is the best all round package out there for speed, reliability, ease of maintenance and low running costs. If your chassis is straight and the engine is relatively fresh, you’ll get lots of fun out of that. If Killo is close to you he will be able to make sure your kart is safe - braking system, steering rose joints, bearings.... and make sure the general kart alignment is good - these Rotax chassis like to twist so it's not unusual to have to realign them every few hours with the help of a scaffold bar.
....Rotax FR125 Electric Start 
