Hello brummum, I’m italian and I know well the 136 Malossi for small frames.
If you want to build a 136 ccm Malossi cilinder on your PK 50’s cases you must change these pieces;
The cilinder, 136 Malossi (but he’s 132 ccm not 136 ccm) direct inlet to cilinder with reed valve or without, or inlet in cases with reed valves or without.
The carburettor DELL’ORTO PHBL 25, if possible with airfilter (but you can mount a PHBL 24, PHBL 22 or SHBC 19/19 - 20/20 always Dell’orto, but I advise a good PHBL 25)
Driving shaft of PK 125 XL (51 mm PK 125 vs 43 mm PK 50)
Flywell PK 50 FL without crown (for the original electric ignition) weight 1,6 kg,
Clutch of 24/58 teeth original ET3/PK 125 or 27/69 straight
Clutch plate special DR with 4 disc
Gear: if you have 4 gear on your PK 50 XL you don’t make anything, it’s good, but I work always with old Vespa motor, (Special and ET3) I don’t know good your motor PK XL 2, excuse me, perhaps I wrong on your gear.
I advise always a 4 gear box, not 3 gear box.
Oh, the pipe, there’s Polini Banana or Leo Vince, but if you want more performance you can buy a Zirri Silent for PK models or Simonini „down and forward“, they costs much more but the increase of performance it’s non comparable to other pipe.
The game it’s done!
I know that Zirri/Simonini pipe costs much more, but I advise for one of these, you spent more money but are money spent very very well…
My last motor ET3 have 136 Malossi direct inlet to cilinder with carbonit reed valve original Malossi, carburettor dell’orto VHSA 32 ES, (with modified to plug/carburettor) driving >or krankt< shaft PK 125 XL, flywell PK FL 1.6 kg, clutch original PK 125/ET3 (24/61), special clutch plate DR, 4 gear box and Scauri’s short 4th gear and Zirri Silent pipe, I cover 300 meters in acceleration in 13,8 (the best) seconds and average to 14 seconds.
The Zirri Silent not have comparison to my old Proma pipe, it’s another planet.