Davids valuation is about right. It's the bracket that is almost inprocurable. A good condition complete unit may bring twice that or significantly more. A great condition mono speedometer will cost less than $100 but has the wrong mount. Greg Henry in Batemans Bay (typarts-aus) has made fantastic twinshock brackets in the past and may have a few remaining. Whilst not original, will do a good job while you hunt.
Maybe use the Trials Australia "Wanted" forum if you are wanting to find something. This is the "Technical help" forum There are also facebook pages for people wanting to buy or sell trials bike parts, such as: Hard To Find Twinshock Trials & Aircooled Mono Vmx vinduro restos and rescue Classic/modern trials bike parts and. sales/wanted Trials Bike Parts for Sale Australia