100 or 120 psi pressure guage from any pool supply or irrigation supply store (approx $5) Just make sure it's a liquid guage instead of an air pressure guage.
Metric adapter kit from AutoZone (under $10)
Use the correct adapter from the kit on the guage - you may have to add in a reducer bushing from the hardware store if the guage you found is bigger/smaller NPT threads than the adapter. Some guages come with 1/8 NPT and some 1/4 NPT. Without walking out to the shop, I think the adapter is 1/8 NPT X M10.
I bought mine from Autozone I think. It runs to 200 psi. You def need higher than 100. On my 01 M900 it will run 100 psi at idle when cold (cold for Florida anyhow). I questioned this but the 01 apparently runs higher pressure.
+1 on the higher pressures. Befoer the bearing started to go, the Monster put out about 70 or so at cold idle and pegged my guage over 100 at anything higher than idle until it warmed up. I just went with the gauge I had on hand already.