MZ/MuZ Riders

Hi,
Hope you can assist. recently bought a barn find ETZ250 that hadnt run for 3 years. after some fettling with the points got it running but there was clear evidence of water in the crankcases and clutch so went for a full rebuild.
So new conrod kit, new piston , seals and bearings and a Tezet ignition/charging system. And now it wont start. checked the timing which was set at 2.5mm btdc, getting a healthy spark, and wetting the plug with fuel but no pop, bang or fart.
when i did a compression check it said 45psi which is way too low, so popped the old piston back in, and i crept to 50 psi .
I now wonder if the conrod kit supplied, might have been wrong, are all 250 conrods the same length?? i thought i got solder squeeze when checking for squish but am now questioning myself.
Was meant to be running at Pendine sands this weekend with it but not if i cant solve this conundrum. Barrel away being rebored to remove that possibility, old clearance was 0.09mm right on the limit of the stated tolerance but it was running before being torn down. I rebuild 2 strokes frequently as would described myself as competent at this task.
any wise words very much appreciated.

Now I am not really sure why some bikes don't like to start, but I just rebuilt an ES250 motor and the reason it wouldn't start (looked at it for hours) was because the crankcase was full of fuel and oil. After I managed to get it all out, the bike actually WANTED to start. The bike also ran before I took it apart, despite 2 of the rings being completely stuck in the compressed position.

I have run an etz250 engine with a piston to clearance bore of 1mm before now, and it still worked!

Well, well. On strip down I removed 115ml of fuel from the crankcases, so carb obviously flooded at somebpoint and fuel didn't leave via overflow but up emulsion tube and into crankcase. Obviously that full the charge couldn't transfer to above piston. Haventvreied starting it yet as snapped one ring and thought I'd do that first. Thanks blurredman same symptom.