@Fearfulsaints in your case, it doesn’t necessarily sound like the SEP itself needs to be different, or at least not that different, just how that SEP is provided.
OT can still be provided even if the setting doesn’t have one on site. This would need to be explicit in F. Not just the direct provision but all the indirect provision and training, etc.
F could include push-pull input describing what the provision is rather than stating the specific activity of chores on the farm. It can still be detailed, specified and quantified rather than vague and woolly.
It could also include having hands on learning. With OT provision embedded. Child-led, if that is what DS requires. Some see child-led as meaning the provision has to be vague and woolly. That really isn’t the case.
Horse-riding could still be included in F even if the setting doesn’t offer it. It could be provided off-site/otherwise than at school/college.
It could include activities of DS’s choosing. Either complete free choice or choice within a pre-defined set of activities. To give you 2 examples of the difference, a WD I am working on at the moment includes detailed, specified and quantified provision for 3 of climbing, horse-riding, swimming, skiing or rebound therapy each week with the activity to be chosen by the YP. Completely enforceable but giving the YP the flexibility to ensure the provision can be responsive to their needs. Whereas, my DS3 has complete free-choice of physical activities detailed, specified and quantified. For 39 weeks per year (the 39 weeks he also attends his AP 2 days a week), he has 3 sports sessions of his choosing per week with each session lasting at least 1 hour. Then he has the same but 5 times per week for the other 13 weeks per year outside of term time. Or your choosing if DS is unable to choose - this applies to my DS1.
One really clear example of how SEP covering environments can be included is where a child requires 1:1 on the school site but 2:1 when in the community. Both situations can have SEP detailed, specified and quantified in F. The same can extend to SEP related to OT.
You might also think about underlying provision. For example, a CYP might not reasonably require 1:1 if they are in a class of 5, but the EHCP might include requiring 1:1 if the child is in a class size of bigger than 6. Another example, DS3 has EOTIS. The tuition element of his package is delivered 1:1 F2F. But he has underlying provision in F related to provision for his APD for if he is ever in a group setting of more than 8. He has some provision for his APD anyway, but the underlying provision provides more. Another example, DS1 has physio in F. He also has rebound therapy, hydrotherapy/swimming and 1 physical activity of our choosing. He has wording that means when he is unable to do rebound therapy, hydrotherapy/swimming and 1 physical activity of our choosing for medical reasons, he has physio sessions instead. All detailed, specified and quantified. All enforceable. The same could apply to some DC with SEP related to OT. Not all, but some.