The MSR Plantation can be built around either a Mamod SL or an MSS locomotive. If you wanted a 'balloon stack', you would need to start with the 'Americanised' SL2 (powder blue) or the SL3 (maroon) engine. The original MSR Plantation had a straight stack. All parts for the Mamod SL range of engines are still available from MSS. So even if your donor engine has some 'issues', you should still be able to sort this with the addition of some new replacement parts.
Your chosen donor engine requires dismantling. Bodywork on Mamod production locos tends to be all rivet construction. These will need to be carefully drilled out. The Plantation loco requires the complete rolling chassis and associated steam parts, smoke box/stack, boiler, a pair of buffers and rear drag beam. Mamod made a kit version of the SL and this makes a good option for a donor engine. Kit assembly instructions are available here and these may also be useful with loco disassembly.