There were hundreds of different types of senders fitted to 4108's. The only compatibility requirement between engine and temperatire sensor is the threading and if needs be adaptors can be used.
Start at the gauge end, who made it and what the spec of the matching sender is. Working back from there there are lots of 3rd party manufacturers of senders that will be able to supply a matching units.
unfortunately all Perkins spares coming via Volvo penta now,a friend of mine who is quite high up in the volvo distribution network told me that Volvo make almost zero profit on their engine sales and look to recover that profit in the spares market!! say no more .
Try here for sensors/senders AND adaptors - Perkins 4108's were/are (IIRC) 5/8 x 18 TPI (UNF) so get an adaptor 5/8 X 18 (Male) to 1/8 NPT (Female) and away you go - senders around £10~12 and an adaptor around £5 - job done for less than £20 and replacement senders are NOT £50 each.
Artificial Intelligence is no match for Natural Stupidity.