You specifically mention Part 135 ops. Do you mean 14CFR Part 135? ie USA. Or one of the other jurisdictions in the world that also name their charter regulations as 'Part 135'**
In the USA, S/E IFR 135 is permitted under 14CFR Part 135. Can be turbine or piston.
When I worked in Oz, the equivalent of 14CFR Part 135 ('Charter' in Oz) under IFR using single engine *turbine* was introduced - but not piston - for pax. However piston IFR charter was fine for freight.
Similarly for the UK when I worked there.
**doesn't mean they're the same rules though.