Tyre sizes varies a little from brand to brand but basically a 120/90x18 would replace a 4.50x18.
A 120/90 is 120mm wide with 90% of 120mm in height. Old school measurement was 4.50" wide by 4.50" high. Newer tyres are lower profile.
Initial low-profile tyres measurements were a little confusing, a 4.10x18 was a 4.00x18 but with 10% lower profile, a 4.60x18 was a 10% lower profile version of a 4.50x18.
