Check the dimensions first on the piston in it but here is a link I found. All the parts on the MW chassis are standard SKM pieces.
http://www.pktaxles.com/mm5/merchant.mvc?Screen=PROD&Store_Code=PKTS&Product_Code=BrakePump22mmRebuildKit&Category_Code=BR
This one from TS racing might work also. there is a rebuild kit link under the picture.
http://www.tsracing.com/store/showdetl.cfm?&DID=7&Product_ID=4266&CATID=41