Norvel engines show up all of the time on e-Bay. Used ones sometimes are reasonable. Bidding on some listed as "New In Box" can get kind of crazy. Sometimes, new parts in the package are listed.
In fact, there is one seller there, Sheldon Hobbies, that has a number of new parts listed. Unfortunately, thay do not currently list a crankcase for the .061. You might try contacting them through e-Bay to see if thay have one for sale. The part number for the Norvel .061 crankcase is 100108-03.
Good luck.
Keith