You can still get the new sleeve 150cc kits from the USA via Ebay ,Dear as poison.
I have found they offer very little if anything over the 3mm overbore kits which equate to 138cc, Use the standard head and base gasket, Do not require fitting new sleeve and machining out of cases to fit and rings are the same as some Chinese kits. Having said that I do have SL 100/125 bores sleeved to 150cc, 162cc 184cc ,and one 99cc stamped SL100 bore at 206cc....