I could be done if several components were adapted to work between the two engines. Wiring looms can be laid over each other and whichever one is needed is the one used.
Realistically though, once you go for the larger power and more reliable engine you won’t look back. Tuck the original parts under the workbench all sealed up and labelled for a future resto - if it ever happens. That’s the plan for my brumby too - upgrade the drivetrain, everything factory that’s removed is kept for future resto options (that will no doubt never happen).