I have seen and read a few posts about this topic. However the part needed always seems to be out of stock. It is for example a TruVative BB032 conversion kit. Can anyone provide more up to date information. I am trying to convert a 10-15 year old Sears bike with a single piece crank to a three piece crank. The chain ring teeth presently seem to be poor quality, although some experts suggest that the appearance is normal. I want to have more freedom to buy modern chain rings as I have been unable to find a double chain ring replacement for the existing Ashtabula type system. The bottom bracket 68 mm long and about 50 mm internal diameter. Will appreciate any help I can get.

This took me about 30 seconds to find:
https://www.bikepartsusa.com/bikeparts/item/01-78753/
and here's a conversion bottom bracket:
https://www.bikepartsusa.com/bikeparts/item/01-98690/

Bikeparts.com has the Truvativ conversion kit which looks really great. ID # BPC110368 for $21. Out of stock, but you could check back if you're set on going to 3-piece.