Hi and welcome to mightyram. The transmissions are THE SAME FOR LHD and RHD. All mechanical part numbers are cross compatible and are the same as well. A quick check on the transmission numbers will verify that. There are rebuild kits for them but they mostly comprise of synchro rings, bearings and seals/gaskets. There are gear cluster shafts as well but I think your issue will be a selector fork/rod issue as tortron has suggested. A bad bearing 'can' cause 4th/5th gear pop outs but it's normally something related to the selector forks or the main gear selector linkage.