Don't Do It, the engines are OK, but the water pump system sucks. I have a 50cc ( 46cc ) C1 Blata clone, and most of the time its apart. Also have an air cooled 39cc Blata clone, in a W/C C1 frame, 13 bikes and 5 project bikes. The C1 W/C is the Worst, the pump is inside the engine cases, and you have to remove the engine and split the cases to fix it, its driven by a O ring belt off the back of the flywheel to the pump pulley under the rope starter housing. The OEM stuff don't last long, and when it fails you don't know till your engine is Fried. Just got mine back together, for the second time, afraid to start it now, although I put SKF bearings, crank and pump and Dupont Viton red water pump shaft seals.
I would look for a 40cc or 50cc, Polini clone engine bike where the pump is on the outside of the engine, I put 3 black lines on the pump pulley so I can tell if its turning. If you can find an air cooled Blata clone, that's ok. I would try to find a W/C blata clone that don't run and convert it to air cooled, the cases are the same, you just leave the water pump off. On my avatar bike made in Italy, GRC GP w/ all alum frame, I cut the body work on the left side near the bottom, so I can see if the pump pulley is turning. It has a 40cc W/C Polini clone engine with a Polini bell and & BZM racing clutch.
Find Kray15's post under Water cooled bike, he will tell you the same story. See photos, first 2 are my W/C C1, the next 2 air the A/C Blata clone, and the last is the 40cc W/C Polini in my avatar bike. I am building a Euro 40cc BZM racing engine as we speak for my avatar bike. Rule #2 Let the Buyer Beware, lol.