The only titanium valved bike I have experience with is the 07 onwards WR450F. The service schedule says check valve clearances every 3rd race or 500 competition kms and inspect valves every 5th race or 1000 competition kms. That's under the most severe race conditions but if you extend that by a factor of 10 for more casual riding, that would mean removing the head for valve inspection every 10,000km.

Similarly for the G450X the valve clearances should be checked after every 20 hours of operation.

I was not aware the WR250R used titanium valves until your post. The little 250 has a couple of factors in it's favour. The valves are smaller and therefore lighter. I'm sure the cams would have less lift than the race 450s, both of these factors would mean softer valves springs and lower seat pressures, which put less stress on the valves. In addition to this Yamaha or someone else may have discovered some magical valve guide and seat materials which drastically increases valve life.

The early Honda CRF450R/X's had a lot of problems with their titanium valves with some lasting as little as 1000km, partly due to side thrust on the valves caused by poor head geometry. It was common to replace these with aftermarket stainless steel valves.

I have researched a bit about G450X stuff because just the other day I was considering converting a G450X into a cross-continent adventure bike. The G450X has a lot of things I like:
-stainless steel frame
-airbox location
-good suspension
-cheap second hand and most seem to have low km courtesy of gentleman riders
-modern 450cc engine design which has the potential to be very fuel efficient
-linkageless direct acting rear shock
-R&V Aqualine make a 12L front and 8L rear tank taking total capacity to 28L.

The big obstacle with this bike is the engine. For reference here is a cutaway diagram of the engine showing that piston, which looks similar to the piston from a formula 1 engine:


To adventurize the engine I would do the following. Costs are my very rough estimates in US dollars:
- Custom 3 ring piston to give CR of about 10.5:1 $500-$1000
- Custom stainless steel valves $1000+
- regrind standard cams with less duration but same lift $500
- Wide ratio 5 speed transmission from Albins Gears $5000
- MicroSquirt ECU $500
- Oil cooler+ tank to increase capacity to at least 4L, $500
- Rebuild engine with new bearings etc + machining + possibly new barrel - $1000-$2000?
- Dyno time $500

Things like the piston and valves are expensive because a lot of the cost goes into the component design and machine setup. If you are buying 4,6 or 8 pistons the cost is amortized over the lot, so the cost per piston is lower.

As you can see nothing is impossible, it just depends if it is worth the cost. It's a totally different proposition for an OEM who can spec the engine before it is built in quantity, in that case it would be cheaper to build than a standard G450X engine. The big question, is CCM having engines built to their specs, or buying a lot of surplus BMW engines?