This page click here tells you what the part number means. I think you will find it difficult to track down a 900MHz Athlon, but there may be some old stock around. I recently burnt out my 850MHz Duron, and had to buy a new motherboard to take a 1.6G processor.
its a Spitfire core AMD and the MS6430 MSI Motherboard in the Tiny 440BXM system will not take the newer core Durons or Athlons I am afraid. Only way is to try and obtain a second hand CPU or rebuild pc in a new case (as they are Tiny) with a new MB.
The suggestion by Kinloss needn't be too expensive. You can get a case for £12,motherboard for £21, 1.6G processor for £25 and heatsink for under £5. You should be able to use all the other parts from the old pc.