Can you give us more input on the RD 40 ?
How slow is slow ? The RD40's weren't famous for running exactly 33 or 45 ?
Is it a DC or AC motor model ?
Can you regulate the speed or is it just on/off ?
Is your belt a flat one or a square one ( you can get new ones e.g. at Ebay )or
Alec's Tip with the oil is a very good one . Have you tried turning the platter without the belt in the bearing ? Does it stop after a few turns ?
If it stops after a few turns I would recommend a proper cleaning and oil refill . If you need material for cleaning and oil you can contact Joel in Germany (he worked 20 years for Thorens but also knows Ariston ) and tell him that you just need the cleaning solution and the oil . His stuff is and works great and prices are fair too. He also has a special oil for the motor itself . See his Ebay offerings for Thorens .http://www.ebay.com/itm/JOEL-S-KIT-THOR ... 591wt_1344
Ariston spindles and bearings are similar to Thorens .
I would also check the capacitor between power and motor .
I would check all this first before considering a new motor as this will be a challenge to find one.
The counterweights in my RD40's are a square piece of led . When you need some dimensions to make one yourself , let me know .