Dunkeswell to Oban and Glenforsa is a long day in a C150. If it's scenery you want the 2 days before the return could be spent based at Glenforsa, fueling at Oban, flying the islands and sealochs. You'd get good information at the Glenforsa Hotel.
Look at the weather before setting off. Sometimes there's a big difference between East and West sides. Consider Perth as a base if necessary due to weather on the west.
Wear lifejackets at all times.
From Glenforsa you could get aerial pics of the Greek style mausoleum on Rhum, the mainline steam locomotive on Soay (off Skye) and the old deisel compressor/generator on the north side of Loch Nevis, well up the hill.