Phil_gretz

Start with the chainline that you want to achieve. Do a google search on "chainline" and you will learn how to measure, and how to determine your optimum.

Next, determine whether your Motobecane frame is of French or Asian manufacture. This will be evidenced by the thread type within your bottom bracket shell.

Take the bike to a qualified local bike shop or coop with trained volunteers, and see if you (they) can find a proper bottom bracket bearing/cone/lockring/spindle setup for your bottom bracket. Best to use loose parts rather than a cartridge bearing, so that you can change out spindle lengths as necessary...

Next comes the empirical part, the spindle length will determine the chainline for your crankset. Doing a search on your particular crankset [may] yield discussions by users who have achieved certain chainline using various spindle lengths. It may not.

If not, you'll need to use trial and error, mounting your drive side crank to a mounted spindle and measuring the resulting chainline. Adjust spindle length to suit your objective - swap out spindles to get the correct length.

If the whole setup is French...good luck finding compatible threaded parts.
If Japanese - you'll have no problems - the coop or LBS will have all you need in their take-off parts bin. Get new bearings, though, and use good quality grease.

PG