Is there a modern cartridge that can be reconfigured to fit a 1905 Manlicher?
7.63x21 Manlicher cartridge. .308 85 grain bullet

Case type
Rimless, straight
Bullet diameter
.308 in (7.8 mm)
Neck diameter
.331 in (8.4 mm)
Base diameter
.332 in (8.4 mm)
Rim diameter
.334 in (8.5 mm)
Rim thickness
.030 in (0.76 mm)
Case length
.84 in (21 mm)
Overall length
1.12 in (28 mm)
Rifling twist
1 in 10
Primer type
B

Cartridges that are very close or can be easily converted:
.32 S&W Long
.30 Super Carry
7.65 French Long

All of these are within a few thousandths of the Mannlicher cartridge with the recent .30 Super Carry virtually identical. It is about .013" short but should be within normal length tolerances. The .32 S&W Long would require lathe work to cut the extractor groove and rim diameter. Dies for the .30 Super Carry or .32 French Long should be appropriate to load the cartridge. The few thousandths difference in diameters would be taken care of in the sizing step if the correct Mannlicher dies can be obtained! These should not be necessary, however.

NOTE: DO NOT attempt to shoot factory .30 Super Carry in the Mannlicher as it is loaded to a chamber pressure of 50,000 PSI and would destroy your pistol. This is for brass to hand load only! If you have a lathe and the skills to use it modifying the .32 S&W Long cases would be the best proposition as it would yield full length cases for this cartridge. Any of these cases are available from Starline, among others.