As everyone else has already stated if the power transformer is designed to run EL34 tubes then you can run KT88/6550 tubes no problem but usually it's the impedance of the output transformer that Marshall had to consider when they began shipping their amps with 6550's instead of EL34's in the 70's. You do have to adjust the bias circuit to provide about 10 more volts of negative bias voltage to run the KT88/6550's in their operating range, there is a tone of info out there on how to adjust the bias circuit from EL34's to KT88/6550's.
That's why Marshall decided to switch to them for the Marshall Major due to the amp 200 watt rating and the 6550's for American market when the EL34's were getting damaged in shipping from the UK to the US as they would work with the impedance range of the output transformers already spec'd out for EL34's. FWIW the KT88 in some opinions sounds and responds kind of like a larger EL34 still with accentuated mids but with more lows and detailed highs.
I'm running a stock Dagnall PT pulled out of a Marshall SLP plexi in my 79 Marshall 2203 and I have been running 6550 tube's for years now, the bias circuit has been adjusted to provide the negative bias range for 6550'/KT88's. I am also running a MM Axiom MM0-100M output transformer in the same amp and I believe this is MM standard Marshall plexi /superlead output transformer which I guess is optimized to run 6l6/6550/KT88 according to MM. But I do believe the stock Marshall plexi output transformer can run KT88/6550's no problem even though the impedance is not optimal for the KT88/6550. I don't think you absolutely have to change out the output transfomer if you don't want to....
Marshall didn't and the KT-88/6550's run just fine in old Marshall's and they should with the O100JM. MM omitted the primary impedance of the MM0-100M on their site for some reason but I did some digging and it appears as though the primary impedance of the MM0-100M and 0100JM both are 1.7K ohms so I don't think you really need to change it out unless you really want to. Maybe someone else can do some sleuthing on the primary impedance of the MM0-100M to see if I am correct or not.
Spec Sheet
MMO-100M
Marshall | 100 Watt | Output Transformer
Optimized for 6L6 / KT88 / 6550 tubes — 1.5″ stack. Uses Grain-Oriented Iron
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Mercury Magnetics
https://www.mercurymagnetics.com › make › marshall
MMO-100M. Marshall | 100 Watt | Output Transformer. Optimized for 6L6 / KT88 ... **
Standard 1.7 kOhm primary impedance version of the TXOP-00001. Bolt hole ...
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The MM O100JM appears to be a dagnal C1998 clone of some sort which should still be able to run KT88's IMHO.
Spec Sheet
O100JM
Marshall | 100 Watt | Output Transformer
Plexi — 1.5″ stack — DE #C 1998 — 1.7 kOhm primary impedance — 3″ x 2 1/2″ bolt hole spacing