You might look into the cost of the replacement part - if it's cheap it would be less hassle.
No reason aluminum would be a problem. Especially as that's an oil fill hole so there's lots of oil mist to keep corrosion minimized.
But getting the part custom -made could be pricey.
Current: 1988 BMW R100GS (the 'numberplate' model)
Past: 1987 Yamaha XT350
1983 Honda CB900F
1980 Honda XL185S
1979 Suzuki GS425E