Standard 20" tyres for L494 are 255/55 R20. These have diameter 789mm and sidewall height 140mm.
275/50 R20 will give diameter 783mm and sidewall 138mm, so close.
Others have fitted 275/55 R20 and claim they have fitted ok, including spare, diameter 810mm, sidewall 151mm

After deciding what tyre size you want, then you will have to find somebody making that tyre size.... your favourite tyre may not be available in the size you want. MY15.5 SDV6 HSE Dynamic without the H . Yulong White