Two things - get yerself a copy of Iain Rice's Loco Chassis book. In the Wild Swan range there are other books about whitemetal and brass kits - even scratchbuilding. Also, I strongly recommend that you watch the BRM DVD's on loco kit building (1,2 and 3). Tony Wright makes it look easy. Of course the monthly magazines frequently feature a kit build - they may have done your model. There are frequently tricky bits that the pros can help you with.