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After cleaning any gasket remains from crankcase and replacing dowels, lightly oil the cylinder and carefully slide into place over the piston (remove the stuffing when the rings are inside the cylinder):
Position head gasket and top dowels:
Bolt head in place (20–22lbf. or 28–30n/m) and replace cam locating bolt (13–16.5 lbf.). Be very careful when replacing the pushrods – angle them toward the outside corner and rock the cam using the kick-start lever to ensure contact. If not positioned correctly they will drop into the motor, necessitating the removal of the head and cylinder in order to retrieve them:
Replace rocker assembly:
Replace engine mounting bolts (8mm 22.5–27.5 n/m)
Adjust valve clearances – intake 0.1mm (0.004˝), exhaust 0.15mm (0.006˝). For correct TDC inlet valve should be closing as piston rises.
Replace tappet cover, refit head steady, carby, spark plug, horn, tank, etc.
Before starting, remove oil bleed screw from rocker cover. Start motor; when oil bleeds from from vent, replace screw.
You won't get much more out of the 150, even if you could find a suitable piston.
The easiest way to get more power is to fit a 250cc clone.
But you would have to do something about the inadequate brakes.
I've just done a 150 conversion on a156fmi, i used the 125 head and cut in the larger 150 valves which i polished and shaped as well as port and polished the head, and would say with mine i had to get the longer 150 push rods to get the rocker angle right.