Smoke oil pumps are usually added to larger aircraft (.90-size +) equipped with 4-stroke engines. The larger airframe size helps absorb the added weight of the smoke oil and smoke oil pump without making the plane too heavy. The 4-stroke engine is required to create hot enough exhaust to activate the smoke oil, 2-strokes tend to run at a cooler operating temperature.

I'd save skywriting attempts for your third or fourth plane, it's quite possible but it isn't for beginners.