Some full size diesels used an exposed element glow plug mounted in the inlet manifold to heat the air in the manifold, rather than inside the cylinder itself. Could you perhaps fit a suitable ‘resistance wire’ element loop across / through the manifold close to the opening, insulated at one end, with the electrical connection, and grounded to manifold at the other? This should be less obtrusive, visually, than an ‘over scale’ plug on a miniature engine. This would also be less susceptible to damage than if inside the cylinder. For test purposes, install the ‘element’ in a short piece of hose to fit over the inlet, before drilling holes, in case it’s unsuccessful.
