This paper explores the potential of leveraging methanol's knock-resistant properties to facilitate both dual fuel (DF) and spark ignition (SI) operation in retrofitted heavy-duty (HD), high-speed marine engines. The study involves retrofitting an original 6-cylinder 7.15L CI diesel engine with port fuel injection (PFI) of methanol to enable DF operation. Later, the diesel injectors were replaced with six spark plugs allowing SI operation. Notably, efforts were made to minimize adaptations to the existing diesel engine, maintaining the compression ratio (CR) at 17.6:1 and retaining the same turbocharging pressure. This research aims to assess the feasibility of retrofitting conventional HD diesel engines (high CR, large bore) for dual-fuel and SI operation on methanol, with a focus on optimizing engine performance, while preserving key characteristics for HD applications, e.g. high torque and high power density. The high CR required spark retarding to prevent knock at higher loads in SI operation. Despite this, efficiencies comparable with diesel were obtained for both diesel-methanol dual-fuel as SI operation on 100% methanol, although differences were noticeable depending on the load. Tests were performed at 1500 rpm with a BMEP of 3.5, 7.1 and 10.6 bar, respectively 22, 44 and 66% of the maximum engine load. The maximum load achievable in stoichiometric SI operation was 12.3 bar BMEP, corresponding to 77% of the original maximum load with diesel CI. At this load, a brake thermal efficiency of 38.5% at stoichiometric conditions was attained. At lean conditions (λ=1.25) an efficiency of 40.1% was reached, with no significant difference compared to a 40.3% efficiency attained in diesel-only operation.