It might pay to get a measuring stick on the transfer port. I've had a few HW35's from this era and the TP can vary in size, even if a leather seal was fitted. I've had some with 2.8mm TP's and the larger 4mm ports; if it was 4mm, you might want to sleeve it down as Geoff has done, but 2.8mm is a straight swap from a leather sealed piston to the newer synthetic seal version. Don't go the route of the steel replacement adaptor on the old leather seal as it increases piston weight and further reduces the stroke of the piston (reducing the available power). A new piston with synthetic seal will give more stroke (approx 4mm) and lighter piston weight, giving a nicer shot cycle. If your TP is 4mm, I'd be tempted to renovate the leather seal on the original piston tbh as they can run very nicely.